I've used this build myself to clear out Honor Mode, it pairs extremely well with buffs like Haste, Bless, Bloodlust Elixir and Potion of Speed to maximize first-round damage, deleting most bosses on Honor mode as fast as possible, even in a single turn but more on that later.

Due to the stacking of Damage Riders (DRS) being changed in Patch 5, it is not as broken as it was before, but still functions as a Piercing-based Bosskiller and there should really only be a single boss in the game that this build can't absolutely destroy and that's an optional boss in Act 3.

Pros:

• Solid from level 1, but level 5 is when this build takes off.
  
• Kills if not severely injures most bosses in a single turn (level 5+)
  
• Very sustainable because you replenish Bardic Inspiration on short rest (level 5+)
  
• Extremely high first-round burst (Building up to 10-15 attacks at level 12)
  
• Solid gear progression with easy to get high performance items.
  
• Can work as a Player Character, with high charisma for dialogue checks.

Cons:

• If something is resistant to piercing, your damage is halved.
  
• Requires highly contested items*
  
• Might require some reloading in act 1**

*Wasn't really a Con for me, since I made this build with Astarion as my main damage dealer.
**Dammon can have hand crossbows in his stock, more on that in the Gearing - Act 1 section.

Race: Any race can pull this off, though Halflings (Lucky) ability can come in handy with the sheer amount of attack rolls you can do, rerolling a 1 is always good.

Origin: Any Origin can respec into this build by following the leveling steps below, but Astarion gets a massive damage increase for this build if you decide to have him ascend as part of his personal questline.

Starting Class: Bard

Starting Cantrips: Pick 2.
Friends: Advantage on social checks is always good.
Blade Ward: Perfect to use before a fight starts.
Mage Hand: Niche, but has its uses.
Light: Portable light source for certain areas, but not 100% needed.

Starting Spells:
Faerie Fire: Best spell in the game (imo) everything hit by this gives Advantage to all attackers, so make sure your allies aren't caught in the area of effect.
Longstrider: Easy set-and-forget buff to increase movement speed if needed.
Sleep: Makes the early game a lot easier by taking entire turns away from your enemy
Speak with Animals: Good for the player character, otherwise not so much.

Starting Instrument: Personal choice, won't matter build-wise.

Background:
Skills gained from Bard: Perception, Deception, Intimidation, Performance
Charlatan: If you're not bringing Astarion, this will let you be the lockpicker, it will give you Proficiency in Deception and Sleight of Hand
Criminal will give you Proficiency in Stealth and Deception
Guild Artisan will give you Proficiency in Insight and Persuasion
From here on, you have 3 Skills left you can pick, I would suggest picking Sleight of Hand and Stealth in case you don't want to use Astarion so you can be the party lockpick but it is really up to you here.

Abilities:
8 Strength
17 Dexterity (or 16 if not using Hag's item bonus)
14 Constitution
8 Intelligence
10 Wisdom
16 Charisma

Level 2 Bard
Learn Spell: Any.
Jack of All Trades - Half your Proficiency Bonus to skills you aren't proficient in.
Song of Rest - An additional short rest that will come in really hand past level 5.

Level 3 Bard
Learn Spell*: Silence to deal with pesky casters / Hold Person / See Invisibility
*Though having another caster to do these spells would be better, you just want to pew pew.
Subclass: College of Swords - For our bread and butter damage: Slashing Flourish (Ranged)
Fighting Style: Two-Weapon Fighting

Level 4 Bard - The POWER level.
Learn Cantrip: Any, you have what you need at this point.
Learn Spell: See level 3.
*If you learned Sleep, you can replace it now if you want, but that's optional.
Feat: Sharpshooter - Your ranged hit chance will plummet, but if you hit you will be destroying whatever you hit. The -5 penalty of this feat will become less noticable over time, especially if you get the Risky Ring ASAP once you hit Act 2!
Also removes your high ground penalties.

Level 5 - The SUSTAIN level.
Learn Spell: Hypnotic Pattern for large AoE Crowd Control if you have a bunch of enemies stacked together or Glyph of Warding for its many different variations of damage and effects.
Font of Inspiration - All your bardic inspiration charges are now regained on short rests instead of long rest, allowing you to blast a lot more high-threat targets in a day.
Bardic Inspiration dice becomes a 1d8.

Level 6
Learn Spell: See level 5
Countercharm - This will cancel out the disadvantage on saving throws against charmed or fightened later, for now it'll give you advantage. Yay.
Extra Attack - This will allow you to attack twice per round, making it possible for you to use Slashing Flourish twice plus an offhand shot for a whopping 5 attacks per round without other sources.

Level 7 - Your First Multiclass
Multiclass into Rogue
Skills: Enhance your dialogue skills if you're the Player Character, otherwise pick whatever you want.

Level 8
Level up your Rogue class
You get a few bonus action variants of Disengage, Hide and Dash. Nothing we actually need.

Level 9
Level up your Rogue class
Subclass: Thief - For an additional bonus action.

Level 10
Level up your Rogue class
Feat: Ability Score Improvement - Picking Dexterity here will let you hit 18 or 20 DEX with the Hag's reward +1 to Dex consumable.

Level 11 Fighter - Your Second Multiclass
Multiclass into Fighter
Second Wind - A small heal that isn't very useful, free potion though.
Fighting Style: Archery - For more hit chance, this will grant you a +2

Level 12 Fighter
Level up your Fighter class
Action Surge - This will give you an extra action once per short rest, capping out on the maximum attacks you can make with this build.

This build will ramp up damage per turn the higher level you are, with breakpoints being level 5, 8 and 12. Note that this is without outside help from Elixirs, Potions of Speed or Haste.
With those you are effectively adding more Slashing Flourish on your first round of combat to deal with whatever Boss or high threat target you want to get out of the way as soon as possible, allowing you to deal as much damage on the first turn with just this high initiative and damage character.
***Note that spaces left empty in the Act 2 and Act 3 sections means there is no replacement item you are required to get in the corresponding act***

Act 1
Weapons: (Ranged Main and Offhand)
Head over to Dammon in the Emerald Grove, there is a chance he sells one-handed crossbows, buy or steal two of these. In case he doesn't sell them, save before you long rest next time and re-check his stock and/or check after every level up to see if he sells them, these will serve you till you reach Grymforge where you can get your offhand best in slot hand crossbow the "Firestoker" which can be found in the dormitory room with the Hellsboars (X:-574, Y:382)

Weapons: (Melee) Just wear at least one melee weapon at all times, preferably Finesse weapons because they use your DEX not STR.
Though you won't really be doing melee attacks unless you have no choice, some passives from daggers can still apply their passive effects.
The Knife of the Undermountain King will apply its critical hit effect to your ranged attacks, making sure your damage roll is rerolled once if you roll 2 or less, this can be bought from A'jak'nir Jeera in Crèche Y'llek.

Helmet: (Optional)
You have a few choices here, each of them has their own use but these aren't entirely necessary:
1) The Haste Helm can be used to gain extra movement speed for 3 turns at the start of combat to position yourself, this is found in the locked chest next to the cart infront of the Blighted Village waypoint
2) The Diadem of Arcane Synergy from Crèche Y'llek can be worn to increase your damage per hit by your CHA modifier, however it will require you to apply a condition to an enemy.

Chest Armor:
At first you want to buy or steal one of the medium armors from Dammon in the Emerald Grove, a +1 version will last you all the way till you're ready to face Grym in the Grymforge where you'll be able to craft a Adamantine Scale Mail, though this is entirely optional because you don't want to be hit in melee at all, but if you find yourself in melee it is a good option.

Cloak:
If you are playing Dark Urge as your main character, the Deathstalker Mantle will actually be very good, granting you invisibility on kill for 2 turns.
If you are not playing as Dark Urge, you can ignore cloaks till Act 3.

Gloves:
The Gloves of Archery are your best pick and can be obtained extremely easily provided you have the gold to buy them, as they are sold by Grat the Trader in the Goblin Camp and will add a flat +2 to your damage per hit for any ranged attacks.
Another option you might want to think about is the Wondrous Gloves from the Mimic in Grymforge, increasing your AC by 1 and giving you 1 more use of Bardic Inspiration but you will not be getting these gloves till level 5-6 at the earliest.

Boots: You have plenty of options to go with but none of them are 100% required for this build, I will list each item you can use depending on what act they are acquired, best to hold on to all of these and swap them around your character(s) if the situation arises.
The Boots of Speed can be acquired from healing Thulla at the Myconid Colony in the Underdark, these will allow you to use the Click Heels ability as a bonus action to double your movement speed and impose disadvantage on attacks of opportunity for one turn.
My personal favorites are the Disintegrating Night Walkers, dropped by Nere in Grymforge, these simply give the biggest amount of bonusses to your Bard, including a bunch of effect immunities and you can't slip on grease or ice.

Amulet: Your best in slot amulet can be found very early on. By killing Kagha you will get the Broodmother's Revenge which gives you a free 1d6 poison damage per hit as long you are healed previously, which is a strict requirement but even having another character throw a cheap potion at your Bard will give this big damage bonus for 3 turns.
There is another amulet in act 2 that can be used for bossfights, more on that in the Act 2 section.

Rings: Act 1 has two rings that are good, any of the rings listed in this guide should be paired with the Risky Ring from Act 2.
The first ring of note is the Caustic Band, a pricy ring that will add +2 Acid Damage to all weapon attacks, this can be bought from Derryth Bonecloak in the Myconid Colony in the Underdark, on Honor mode, this goes for a hefty price.
The second ring of note you can get from an elegant chest in Inquisitor's Chamber, Crèche Y'llek, the Strange Conduit Ring will add 1d4 points of Psychic damage to your weapon attacks as long as you are concentrating on a spell.

Act 2 will provide you with a variety of equipment improvements, including getting your best main hand weapon and ring for optimal damage per turn.
Though leveling all the way to level 12 will still give you good bonusses at certain thresholds, Act 2 is when you can really get a lot of good gear for this build, with only a few items from act 3 rounding out everything.
One thing of note is that all your bestarmor is in Act 3, so finding something that will either buff up your AC the most or give you the best utility will help your bard to survive in Act 2.

Weapons: (Main and Offhand)
The Hellfire Hand Crossbow is your best in slot mainhand is found in the Gauntlet of Shar and is dropped by Yurgir upon death**
**Killing Yurgir will mean you have both your weapons for the endgame, but Yurgir can be a nasty surprise on Honor Mode, so if you want to beeline for him make sure to bring invisibility countermeasures.

Chest Armor:
The Yuan-Ti Scale Mail is considered the best Act 2 chest armor for DEX characters, but if you have someone in melee range, you're going to want to give this to them instead.
This armor applies your full DEX modifier to AC and give you +1 to initiative.

Helmet:
The Covert Cowl is dropped by one of the Meenlocks found in the Last Light Inn Cellar, this provides the same critical hit range increase bonus that your best in slot helmet does as long as you are obscured, which isn't that hard to keep up.

Gloves:
The Flawed Helldusk Gloves crafted by Dammon in the Last Light Inn if given an Infernal Iron is a optional replacement to get 1d4 fire damage, but if you have an unarmed character you might want to pass this off to them and get the Dark Justiciar Gauntlets instead, replacing your previous gloves +2 damage with 1d4 necrotic damage

Boots:
Mattis at the Last Light Inn sells the Evasive Shoes, which are a simple flat +1 to Acrobatics and AC, in case you need more AC and these aren't being used on a frontline character.
If you decide to kill the Nightsong, the Dark Justiciar Boots will give you a Misty Step-like ability.

Amulet:
The best humanoid disabling amulet can be found from Malus Thorm at the House of Healing, the Surgeon's Subjugation Amulet will allow you to Paralyse any humanoid on crit once per long rest, this will flat out disable any single humanoid for two turns with no save and is extremely powerful on this build because we're stacking items that will make out crit chance better.

Rings:
Your first best in slot ring is the Risky Ring, which can be acquired as soon as you have access to Moonrise Towers, as it is sold by Araj Oblodra, this ring will give you advantage on all attack rolls at the cost of disadvantage on all saving throws, but will make it even more likely for you to crit. Note that this is the best ring in the game for characters that like to attack a lot and pairs extremely well with builds just like this that like to attack way too many times.
Another option for your second ring slot is found in a opulent chest in the vault room near Balthazar, inside the Gauntlet of Shar: The Callous Glow Ring's requirement is that your target needs to be illuminated and give you +2 radiant damage against such targets.

Act 3 Part One
Act 3 is split up in 2 parts with the second part of gearing up listing items obtained from some of the harder bosses in the game.
I would recommend to only read Part Two of this section after you hit level 12, giving you all the character abilities, features and spells at your disposal, making beating each boss much easier.

Chest Armor: All these of the following armors can be used but from best to worst these are:
The Armour of Agility is the best armor of any DEX character in the game and can be picked up by beelining to Gloomy Fentonson at Stormshore Armoury in the Lower City.
Your second option is the Elven Chain sold by Exxvikyap in Rivington, though your AC won't be as high as your other two options, it will give you advantage on DEX saving throws and +2 to Initiative.
Your last option is the Unwanted Masterwork Scalemail, sold by Dammon at the Forge of the Nine provided he is still alive. This armor allows you to add your full DEX modifier to AC, providing resistance to fire damage and you won't have disadvantage on stealth checks.

Cloak:
The best cloak in the game for ambushing is the Shade-Slayer Cloak, which is very easy to get because it is sold by Sticky Dondo in the Guildhall, found in the Lower City Sewers.
This will lower the number you need to crit by an additional 1 as long as you're hiding which is perfect if you don't care for dialogue.

Gloves: (Optional)
If you require more burst the Martial Exertion Gloves will let you take 6d6 points of piercing damage in exchange for another attack and double your movement per short rest. These are sold by Bumpnagel in Angleiron's Cellar, accessible via a cave entrance at (X: 30 Y: 29) in Rivington.

Boots:
The Boots of Very Fast Blinking are a reward from Akabi, the Djinni at the Circus of the Last Days in Rivington but it is very RNG as it is one of the 16 consolation prizes he can give out but each try will cost you 500 gold.
The Spaceshunt Boots dropped from Dolor in the Lower City as part of the Murder Tribunal quest is better than the Misty Step boots, but there's not much of a reason to use them in combat.


Act 3 Part Two
Now that you hit Level 12 you have all your abilities and can more easily deal with the hardest encounters in the game.

Weapons: (Melee Main and Offhand)
Your best in slot offhand dagger is dropped by Orin, another mandatory bossfight, her Bloodthirst dagger further reduces the number you need to roll a crit by one.
Important note: At this point you should put the Knife of the Undermountain King in your main hand and the Bloodthirst dagger in your offhand, if anything comes too close to your Bard, simply attack them with the offhand dagger to inflict piercing vulnerability to whatever poor soul thought it was a good idea to come close for an insane amount of increased damage.

Helmet:
Sarevok's Horned Helmet Lowers your critical hit range and with the advantage of the Risky Ring will make it so you'll crit even more often.

Gloves:
By far the best gloves in the game for any damage dealer are the Helldusk Gloves, dropped by Haarlep in the House of Hope and you won't be acquiring these unless you can deal with the hardest bossfight in the game.
Legacy of the Masters is a close second, with a +2 to attack and damage, ontop of being much easier to acquire, if you're not planning on getting the Helldusk gloves, settle for these as they can be bought from Dammon in Act 3, provided he is still alive.

Here is a quick overview of the best in slot gear for this build.
If another character in your party has a better use for some of the gear just follow the list to find your next best alternative.

Melee Weapons:
Mainhand: Knife of the Undermountain King
Offhand: Bloodthirst

Ranged Weapons:
Mainhand: Hellfire Hand Crossbow
Offhand: Firestoker

Helmet: Sarevok's Horned Helmet > Covert Cowl

Cloak: Shade-Slayer Cloak

Chest: Armour of Agility > Elven Chain > Unwanted Masterwork Scalemail

Gloves: Helldusk Gloves > Legacy of the Masters > Martial Exertion Gloves / Flawed Helldusk Gloves / Dark Justiciar Gauntlets

Boots: Preference, but I would say Disintegrating Night Walkers

Amulet: Broodmother's Revenge / Surgeon's Subjugation Amulet

Ring 1: Risky Ring
Ring 2: Caustic Band > Strange Conduit Ring / The Callous Glow Ring

Elixirs and potions are pretty optional until the end game where you can get the maximum amount of efficiency out of them, your most notable consumables are:

Elixir of Bloodlust: This I would herald as your best Elixir as you most often will be dealing with multiple enemies and getting an action once per turn on kill will make it so that you can oneshot a weak enemy while keeping the same amount of damage on a boss every turn.

Elixir of Viciousness: If you're not fighting a boss, are out of Bloodlust elixirs or simply have too many to spare, these are your friend, quite easily available starting in Act 2 and you don't need these on this bard build in act 3.

Potion of Speed: Being a potion, this does not overwrite Elixirs, so this can be paired with both the Elixir of Viciousness or the Elixir of Bloodlust to give you: +2 AC, advantage of DEX saving throws, double movement speed and an additional action.
Perfect for boss killing, but do note that after 3 turns, you are unable to take any actions for one turn.

Simply said, this is a section to list some potentially useful items that can be used by other characters, I won't go into other build details but I will explain shortly what each of these items will do for your Bard.

Act 1: Phalar Aluve - Found stuck in a rock in the Underdark (DC 15 STR or Religion check)
If you have a frontliner, this sword has a special ability called Shriek, when activated this will make anyone surrounding the wielder take 1d4 additional thunder damage to any creatures and reduce ALL their saving throws by 1d4.

Act 3:
Bhaalist Armor - Sold by Echo of Abazigal after becoming a Unholy Assassin.
This Light Armor will make anyone within 2m / 6.5ft become vulnerable to piercing damage unless they are resistant or immune and will make your DPS skyrocket.

I was a big fan of Larian Studios, being one of the people who absolutely loved playing Divinity: Original Sin 2, many thanks to Larian Studios for creating probably my game of the decade.

This guide would not have been as easy to write without having the BG3 wiki, which gave me easy access to all the item descriptions and locations I needed to write this guide.

Many thanks to the many Bard class builders who I've taken all this information from, with a special shout out to cRPGbro and Werglia, some of my favorite RPG youtubers that create lots and lots of builds for Baldur's Gate 3 and other games in the genre.