Alatreon is my favorite fight in Monster Hunter right now. I know, she's a salt mine that seems to have annoyed or exhausted just about everyone in the MHW community. This is a true challenge, that requires knowledge of her moveset, as well as knowledge of your weapon and the special mechanics of the fight. This guide solely covers the use of the Switch Axe, my main weapon. It's a very dynamic fight that remains engaging throughout and there is barely a moment to rest. At the same time, every single attack in Alatreon's moveset is avoidable and punishable with decent positioning and reactions.

That is why, far and away the most vital skills for the fight before anything else are Evade Window 5, Evade Extender 3, and Elemental Attack 6. The build is contained below, but just know this is not really going to resemble 'meta' sets, and this setup is build specifically for Alatreon. If you follow this guide and spend some time getting to know Alatreon, you're going to kill her. Here is a video of me hunting her using this build, and while my play is pretty sloppy (with some embarrassing axe reloads) I don't even cart a single time. You don't have to be superhuman to beat this Elder Dragon!

Before anything else, this can be easily adapted to any element, just switch out your charm and a single decoration slot. Everything else is good to go. If you don't have the Enhancer/Vitality jewels, you can use something else without too much trouble, though power prolonger does make the fight more forgiving since you don't have to charge your sword form up as much.

Why Namielle?
The Namielle Gamma set is the easiest way to get the vital Evade Window 5 and Evade Extender 3 skill points, as with the Safi helm, gloves, and legs, you get all of that with no need to use decoration slots. Most meta elemental builds use all five pieces of Safi'Jiva's armor, and while that will technically get you more damage, you'd have to sacrifice the evasion. Since you won't be able to roll through attacks to stay on offense, you'll actually be doing significantly less damage than if you just took the evasion. The Namielle set bonus is also a little icing on the cake and covers some for the loss of elemental damage Safi 5 would normally give.

EDIT: You can still go Safi 5 if you want to just gem in the evasion skills, but that takes a pretty heavy investment and you won't have as much room for damage. Overall I think it about evens out, but if you really know her moveset feel free to go Safi 5.

Blight Resistance?
You don't actually need this skill if you're really good, but all of Alatreon's quicker attacks will apply Dragonblight to you if they land, which makes the fight a lot harder. I'd recommend taking this skill regardless if you have the resistor/expert jewels. If not, I'd actually advise slotting out the expert in favor of resistor. You'll do enough damage, don't you worry.

No Charger?
It's not commonly known among Switch Axe users that have only ever used power phials, but power element phials actually charge up extremely fast. You won't need charger as your B, B combo will usually fill the gauge all on its own.

Power Prolonger!
I know Enhancer/Vitality jewels are really rare, but it's my favorite skill for Switch Axe. Not only does it extend the duration of your sword charge, it keeps you in Power Axe longer which is actually helpful for this fight.

What Element do I Use?
There are a couple of options. If you're really good at breaking the horn, use Ice or Fire. If you're less confident that you can do that, use Water or Thunder. I have had no luck disrupting her elements with Dragon.Only take Ice or Water if she starts the quest in her Fire form, and only take Fire or Thunder if she starts the quest in her Ice form. The Special Assignment Alatreon always starts in Fire form, and you'll have to read the description for the Event Quest to know which one she begins as.

EDIT: I have disrupted her with dragon element using the Alatreon Switch Axe. It has such an absurdly high elemental damage you can actually fight her the whole time in axe mode.

I don't have Kjarr weapons!
EDIT: You don't need Kjarr weapons. They're the best option for pumping out elemental damage, but not totally necessary. If you don't have them, I'd recommend using the Deep Lagoon II, Lohenbiel, Archbeast Kirin, Hector Glacia, or Safi elemental axes. No endgame weapons are even needed for this fight.

Her Elements
Like I said before, Alatreon will always start in either Fire or Ice form. Throughout the fight, she will also switch to a Dragon form. The three forms' weaknesses are as follows:

Fire: Very weak to Ice, weak to water, immune to fire.
Ice: Very weak to fire, weak to thunder, immune to ice.
Dragon: Somewhat weak to dragon, but vulnerable to all elements.

As you can see, Ice and Fire are both extremely effective elements against her assuming she's in the right form. Thunder and Water are more consistent because they're effective against all three forms with a decently large boost against Ice and Fire forms respectively. As such, I recommend Ice and Fire once you're confident in fighting her, and Thunder and Water for if you're less confident.

Manipulating Her Elements
While she is in Dragon form, you can break her horn. If you do that, she will be unable to change elements to either Fire or Ice form depending on which she was in most recently. If she begins the fight in Fire form, then changes to Dragon, and you break her horn? She will be unable to use Ice form, and will revert to Fire form next time she switches elements. This is a very important mechanic for if you use Fire or Ice weapons, as you can avoid her going into a form that's immune to your element. It's also a decent damage boost if you use Thunder or Water, but isn't totally necessary.

If you want to use Fire or Thunder weapons, break her horn to prevent her from using Ice form.
If you want to use Ice or Water weapons, break her horn to prevent her from using Fire form.
If you mess up and can't break her horn, then try to avoid breaking her horn next time she's in Dragon form so she'll switch back to the element you're good against.

An additional note, you can only break her horn while she is in Dragon form. It's the most elementally vulnerable part of her body in general though, so still hit it when possible but don't worry too much about it while she's in Ice/Fire Forms.

EDIT: Her forelegs take the most elemental damage. Hit them until you disrupt her, that's more important than breaking her horn.

ESCATON JUDGMENT!:
This is her signature move. This is what wipes parties and why I only hunt her solo. It will kill you if you aren't good enough, that's why it's called Judgment. It's an immense nova that can't be avoided and drops your HP in an instant.

Treat her Dragon form as a halfway point. When she switches to Dragon, she's halfway to using ESCATON JUDGMENT!

There's a way to stop it from happening, though:

Elemental Disruption
If you deal enough elemental damage to Alatreon, she'll topple to the ground and the Handler will say you've done a good job controlling her elements. This will significantly reduce the damage of her next ESCATON JUDGMENT! and allow you to survive it.

It will still deal heavy damage, so preferably sit in a health booster and chug a mega potion while it hits you, eat an Astera Jerky, or pop a max potion. Anything that heals you a decent amount. I prefer Astera Jerky, and you maximize its healing if you pop it right before your health hits 50%. Make sure you're at 100% HP when the move starts, though. You don't need the health booster if you're really good at timing the Jerky.

You can actually disrupt her elements more than once before ESCATON JUDGMENT!, and it will reduce the damage even further. I do so in the video posted at the start. Disrupting her elements is the most important part of the fight unless you intend to just tank it and hope you can kill her before she gets 3-4 knockouts on you.

With the Switch Axe, the most important hitzones for her are the forelegs and the head. The forelegs are the easiest to pile damage onto but the head should be focused whenever she's been toppled or she uses an attack that makes it vulnerable.

EDIT: Only go for the head when she's in Dragon mode.

Her hindlegs are the safest part of her body to stand next to, though, and are a great place to charge your sword using your B, B combo before moving on to hit her forelegs or head. Really, just hitting her whererever you can works just fine for the most part whenever she's in Ice or Fire form.

Once you've disrupted her elements and/or she's gone into Dragon form, go for her head to break that horn! It's actually pretty easy to break if you can get a few good combos on it. Using your rising slash and overhead slash in axe form is the best way to smack her noggin after lighting attacks. Especially if you're in Power Axe.

Zero Sum Discharge?!
I know a lot of people use Switch Axe as a bot to spam ZSD over and over again using temporal and rocksteady mantles, but that's not going to work at all. ZSD should be reserved for moments that you absolutely know it will pay off, and also saved for her Dragon form. I wouldn't even put on the Temporal Mantle except when she's in Dragon form.

The main reason ZSDing her face should be done with caution is because of her lightning attacks. All her lightning attacks cause her horns to crackle and zap you if you're attached to them, and it will eat through your entire Temporal Mantle in an instant. Notably, she has no access to lightning moves while she's flying, so while it's normally a difficult time to hit her, you can use the temporal mantle and ZSD on her face while flying for really solid damage. But again, save this for Dragon form to hit her horn.

If her horn's already broken, and you still have sword charge and she's flying, ZSDing her tail is a good way to get some damage off on her, though I would recommend immediately canceling to the small explosion to put yourself at minimum risk.

Bonk?
No. Don't worry about wallbonking her. She spends most of the fight enraged, there are only a few spots you even can flinch shot her, and all of her attacks use her head so it's incredibly unsafe and not worth baiting her for. You actually can go for a flinch shot while she's flying, though, assuming you have temporal mantle up and she's in Dragon form.

Soften?
Don't worry about softening either. It's similarly unsafe to flinch shotting for the above reasons and should only be done when she enters the drooling animation. Even then, when she drools for a Clutch Claw, if you have sword charge it's going to be better for you to just clutch into ZSD. The clutch claw should only be used sparingly in this fight, you can't just bully Alatreon with it like most of Iceborne.

Forward fire beam: Alatreon breathes fire in a forward beam. Just roll to the side. If you're close enough, get a hit on her face with rising slash.

Spinny Fire/Water Beam: Alatreon flies up in the air while spitting fire. Roll through it. This build is Evade 5 for a reason.

Sweeping Fire Beam: Alatreon breathes a beam of fire to her right and sweeps to her left slowly. Roll through it, or close the distance to get good damage off on her forelegs.

Forward Fire Shockwave: Alatreon flies backwards while doing this one, so you can't really punish
her for it, but if you roll towards her you'll usually fully avoid it.

Big Fire Shockwave: Alatreon spits out a ball of fire that creates two shockwaves. The first is in the center of the ring, the second is the exterior. Roll out of the center, then roll back into the center and it will put you right next to her head for some good damage! If you aren't positioned right, just roll out of the whole zone.

Downward Fire breath: Alatreon flies up and breathes fire straight downward, dealing damage over time in a decent AoE, then dramatically lands in the center and roars. Roll out of this one right when it starts, then roll back in just as Alatreon lands to get some hits in.

Downward Ice breath: Same as above, just colder.

Water/Ice bolts: Dodge 'em. She only does them when she's a fair distance away from you and they're usually pretty easy to see coming. The forward ones are harder to dodge but it's not a big damage move or anything. She also tends to use this move only while flying.

Ice Rain: Alatreon begins breathing ice, sweeps around herself, then flies up in the air and breathes ice into the sky, which after a long delay will cause ice to rain down. THIS IS HER SLOWEST MOVE. Hit her during the startup, and you can use the time she's in the air with it to sharpen fairly safely.

Cone of Cold: Alatreon breathes out a wide cone of ice, dealing damage over time while you're in it. It's a very slow move, so roll out of it and towards her. You can get good hits on her head or forelegs during this one.

Lightning Rain: Alatreon stands up on her hindlegs and summons lightning, either starting near her and travelling out, or starting distant and travelling in. Either way you're very safe near her hind legs so go to down with the damage, use them to charge your sword, heal, sharpen, whatever. Just use the sword mode hop to reposition if one is going to hit you. Count yourself lucky when she uses this attack a lot. Be a little careful as it ends, as she likes to use Bodyslam right after this move.

EDIT: You can tell if the lightning will start close or far by the direction the sparks go in while she's charging. If they're going toward her, the lightning will start far. If they're going awa, they'll start close.

Lightning lines: Alatreon copies Kirin and calls down lines of telegraphed lighting. Don't pre-emptively dodge as she can change the angle between each use. She'll do it either two or three times. You can fairly safely attack her a little too while she's doing it.

Advancing Lightning: Alatreon calls down a big wave of lightning that starts near her and advances outward. It's super easy to just roll forward through, and she'll roar after it starts. This gives you plenty of time to bop her noggin in axe mode.

Dragon Rush: Alatreon copies Kushala Daora and runs, dealing dragon damage. It deals very little damage and we have blight resistance so just sidestep it. She'll sometimes follow up with a second one, so watch out for that.

Dragon Dive: Alatreon charges up with dragon energy and flies at you, crashing into the ground. This one's hard to avoid but try and roll to the side.

Dragon Nova: Alatreon charges up with dragon energy, then plants her hands on the ground, attacking in an AoE. With evade 5 we can easily roll through this, time it so you roll right as her forelegs hit the ground and you can get some good attacks in. This is what she uses to switch to Dragon form.

Flying Dragon Nova: Like above but Alatreon flies towards you to do it. She only does this move in the air and its timing is a bit different. You can't really punish it so just be on your toes and Superman dive if you want to.

Body Slam: Alatreon rises up and slams the ground, dealing dragon damage and knocking you onto your back. It's a scary move, but once you're used to it you can roll through it to immediately begin hitting her. In sword form you can even just sidestep into it and abuse her.

Claw Swipe: Alatreon swipes a claw dealing small damage. Just roll towards her and keep hitting her. She usually only does this while you're in front of her.

Headbutt: I hate this attack. She sweeps her head twice near her and it does a lot of damage for how fast it is. If she's in the air when she does it, she follows up with a tail swipe that's easily dodgeable.

Tail Whips: it's Lunastra's tail whip, but Alatreon's chonky tail is too slow to really hurt you and you can just hit her. She usually only does this in multiplayer or if your Palico is behind her, though.

Flying Fire/Water breath: It's like Teostra and Lunastra's flying fire breath but somehow less scary? Don't worry about it.

You don't need many items for this fight. I recommend Mega potions, Max potions, Ancient potion, Astera Jerky, and Whetfish Fin+. That's it. Astera Jerky is for surviving Escaton Judgment! once she's disrupted. Whetfish Fin+ is just better than whetstones, use them. Use them Use them Use them. There's no good time to sharpen without them. For Palico, go with Vigor Wasp and a sleep weapon. You probably won't get a sleep off on Alatreon, but hey, it's free damage if you do. I also believe that Mascarpone Fudgement! will still kill you through vigorwasp revival, but I'm not certain.

EDIT: Nope, it kills you regardless of vigorwasp revival.

Finally, remember to eat for Felyne Safeguard if you're unsure of yourself or Elemental Defense Up (L) if you are confident.

The last thing I'd like to say, there's a lot of complaining about the "Elemental DPS Check" being artificial difficulty, but it's just there to encourage you to play well. You can't go back for items without carting, and you can't just brute force your way through (easily at least, lol). You can't turtle up and rely on other people to carry you. The "DPS Check" isn't nearly as high as people would have you believe, in the video at the start I get a disruption off on Alatreon using Thunder, not even the element she's weakest to, in the first MINUTE of the fight, and a second time later. It's really not that bad. If you fail it, it's because you're getting hit and healing too much, or failing to exploit openings. Sure she sometimes flies around a lot, but that's by no means for the entire fight. Really, just focus on playing well and memorizing her moves. You can do it!

Oh, and this guide only applies to Switch Axes. I think that other weapon types are a lot more justified in their annoyance.