DISCLAIMER:
If you find the boss too hard, understand first that this is my first time making a boss fight (Actually, this is my first time coding NPC AI in general...), so I can't guarantee a flawless boss design and balance.
I will also note that I have personally verified the boss fight is possible in all vanilla difficulties. This includes pre-Plantera, Legendary Get Fixed Boi, whilst the boss was enraged (As a summoner, if that counts for anything in your books).

If you find the boss too easy, refer to the first point above. And yeah, circles go a very long way in this fight lol. Still, I enjoyed making and fighting the Core, so hopefully you will too. But if you're having a Bad Time, that's what this post is here to help with.

Though the boss is available post-Mechs, it is generally better to wait to fight the boss until post-Plantera due to its large HP pool and aggressive AI.

Utilizing good wings (And a dashing accessory in Expert+) to perform a fast circling motion around the boss will trivialize dodging most of its attacks. This does not work for Night Bombs or Cursed Storm.
On a similar note, avoid moving in a single direction for too long, as the Core will likely outrun you.

If you don't want to dig out an arena underground, the Core’s enraged state isn’t drastically stronger, meaning it’s a perfectly viable alternative to fight the Core enraged with a larger arena on the surface. Its boosted speed will likely demand a fast circling motion to dodge it, though

Weapons with AoE should be utilized against the Orbitals, since they appear in groups.
Meanwhile homing weapons are very useful against the Core itself, since it is a small, singular target that moves quickly.

Similarly to Skeletron’s headspin, the Core can stunlock the player by hovering on top of them. Furthermore, if the player lacks knockback immunity, they can get hit back and forth between the Core and its Orbitals. Tools such as a hook, Slimy Saddle or Gelatinous Pillion, or Rod of Discord can help the player to escape.

Note that this walkthrough will assume you are fighting the boss ASAP; pre-Plantera. Using post-Plantera gear will make the fight much easier
It will also assume you know basic information, like how to spawn the boss. There is another Psuedo-Wiki post with this information and much more, it'll come in handy.

- Chlorophyte Armor is generally the best choice for most classes. Its crystal leaf is very useful
- Hallowed Armor is a viable alternative for its set bonus and decent stats

- Melee players who prefer defensive sets can use Turtle Armor
- Summoners who like defense, mutliclassing, or sentries can also use the T2 OOA armor sets
- Spider Armor has better offensive stats for aggressive summoners

Melee:
- The Death Sickle's piercing and lingering projectiles make the best all around choice
- The True Night's Edge is a good alternative, but is very hard to aim
- The True Excalibur is also strong, but very risky due to its shorter reach
- Light Discs offer a higher range, less risky method of attack
- Note that the Fetid Baghnakhs are NOT very viable due to the Orbitals hovering out of reach

Ranged:
- The Flamethrower decimates the Core, Orbitals, and Shade Wisps. It's also easy to hit with
- The Onyx Blaster offers solid AoE damage
- High Velocity, Exploding, and Crystal Bullets are all good ammo choices for any gun
- Chlorophyte Bullets are particularly useful for the Core, since it's small and fast
- The Dart Rifle/Pistol when used with Crystal Darts can bounce projectiles between Orbitals
- A Chlorophyte Shotbow with Chlorophyte Arrows allows easy hits in enclosed arenas
- The Super Star Shooter's piercing is very powerful

Mage:
- The Rainbow Rod has great AoE and homing capabilities for all parts of the fight
- The Venom Staff's multiple, piercing projectiles make it good for attacking the Orbitals
- The Magic Harp's lingering, bouncing projectiles excel in enclosed arenas
- Life Drain can be used on Shade Wisps to get a ton of life regen
- The Nimbus Rod and/or Clinger staff is good support for ALL classes

Summoner:
- The Sanguine Staff and Blade Staff are both very reliable
- Whips are excellent for their AoE, and synergize especially well with the Blade Staff
- The Optic Staff has strong damage potential against the Orbitals
- A Queen Spider or T2 OOA sentry is a useful supplement to any build

- As ever, bring your strongest Healing Potions (And Mana Potions, if playing mage)
- Ironskin, Endurance, Regeneration, and Lifeforce are all great for survivability
- Any food item will grant nice, all-around boosts, plus much-needed life regen in Expert+
- Wrath and Rage potions are great for increasing damage output
- Melee users benefit from ale
- Rangers using a bow should bring a Archery Potions
- Magic Power and especially Mana Regeneration Potions are necessities for Mages
- Summoning Potions help any class, and are particularly beneficial to Summoners

- A strong pair of wings facilities circling the boss, which trivializes many of its attacks
- Amphibian Boots are an excellent supplement to wings, and make circling much easier
- Some form of knockback immunity - such as the Ankh Shield - helps avoid being stunlocked
- The Charm of Myths and/or Cross Necklace (and upgrades) greatly boosts survivability
- Class Emblems (and upgrades) are good for an offensive upgrade
- In Expert mode or higher, a dashing accessory (Ex: SoC) is a must-have

- It's recommended to max out HP to 500 via. Life Fruit at this point since the boss is post-Mechs
- Gathering permanent upgrades from shimmered items like the Vital Crystal and Aegis Fruit are a good idea, too
- The Gelatinous Pillion mount's vertical mobility is excellent for quick escapes
- The Rod of Discord is very useful for escaping stunlock scenarios
- A Hook is a viable alternative to the RoD in the noted use-case. The faster it reels, the better.

As ever, campfires, heart lanterns, and the like are all good additions to your arena

You can fight the Core either 'naturally' in the Undergound Corruption/Crimson, or spawn it manually to fight it above-ground or elsewhere. Arena considerations change based on this choice, as detailed below;

Your arena should ideally be around the size of a screen (100+ by 60+ tiles). Widening Marble or Granite caves or utilizing a Glowing Mushroom area is a great place to start.

Have a layer of platforms or two, but leave some (horizontal) gaps between platforms on each layer so you have spaces you can freely fall through.

Peace Candles are particularly helpful while building, given the high spawn rates of dangerous enemies in the Underground Corruption/Crimson. However, once the arena is complete you may want to turn them off to encourage the spawning of Suspicious Looking Orbs.

'Enraged' should be taken lightly; it is only a mild enrage, intended to merely accommodate for the increased space allowance. It is perfectly viable to fight the boss enraged. In fact, I (BlockCrusder) personally prefer the enraged fight.

If you can wait to summon the boss at night, you'll be able to spawn it anywhere. This means you can use an existing arena you built for a previous boss fight.
If you'd like to summon it at anytime, you'll have to do so at The Corruption. In this case it may be in your interest to set up a simple arena on-site or nearby.

Since you'll have plenty of room to fly about on the surface, all you'll really need is a few spaced out platforms to reset flight time when you need it.

As noted in the basic tips section, a circling motion is an extremely potent method for dodging the boss. This requires strong wings, as well as a dashing accessory in Expert+, but is a highly recommended approach.

To progress the fight, you must destroy the Core's Orbitals. It will begin with 3 Corrupt Orbitals, and later in the fight summon a second wave of 4 Cursed Orbitals. Weapons with strong AoE - such as the Flamethrower - are best suited for this task. Circle the boss aggressively to evade its constant pursuit. If an attack cuts off your path of travel, begin circling in the other direction, for a figure 8 style of movement.

Be wary of the Night Bombs and Cursed Storm attacks; circling will not work on these since they strike in all directions. If possible, distance yourself from the boss/projectiles to either out-range the attack or make it easier to weave through it.

Meanwhile, Shade Wisps aren't too dangerous, but function as an annoyance that will soak up some hits. It's generally best to just go ahead and dispose of them, lest one fly in at an odd angle and catch you off guard.

Shadow Blasts, though dodge-able with circles, are still very dangerous since the Core will speed up before attacking. You'll have to move extra aggressively to stay out of range. The Gelatinous Pillion is incredibly useful here.

Once any living Orbitals are down, you'll be able to damage the Core. Homing weapons shine here, since the Core is small and fast moving. The boss becomes faster and more aggressive, so you'll probably have to step up your circling to stay on top of things.

Normally, the boss only has two special attacks here - the Dash and Cursed Storm V2 - but both cannot be dodged with circles. Once again, distancing yourself before the attack is very useful.
For the Dash attack, once you see the charge begin, make a quick turn to avoid it. Note that this is especially hard to time in Expert+, because the Core will predict your movement.
Cursed Storm V2 is accompanied by Shade Wisps. As before, it's best to take these out when you have the chance.

In Expert mode, the Core gains two more special attacks - Bomb Chase and Cursed Barrage - which CAN be circled.
Note that Bomb Chase is particularly chaotic since the homing Night Bombs can be launched in any direction. Be prepared to quickly change direction!
Cursed Barrage will spawn Shade Wisps; you know the drill. Other than that it's rather unremarkable.

At 5/9s HP, the Core will revert to its Invulnerable Phase as it will spawn its second wave of Orbitals. From there just rinse and repeat, albeit at a faster pace since the Core has an extra Orbital and will also move a bit faster.

Good luck!