Alright, before we get started, my build is pretty much moulded by this post here. Shout out to Dagrix for the build (the link has a full guide which I didn't get to see until about 15 or so losses in T_T)
w.reddit.com/r/MonsterHunterMeta/comments/hq7dqw/a_complete_detailed_alatreon_guide_for_lance/
I personally had seraphic0's video for aid.
Now this set is the one that got me to the dance. Of course, replace Kjarr Crest King with Kjarr Crest Stream and replace blaze jewels with frost jewels if you're doing the Special Assignment or Event quest (aka 'The Evening Star') and replace Kjarr Crest King with Kjarr Crest Stream.
Skills
View media item 33411View media item 33412Decorations
View media item 33413Weapon Custom Upgrades
View media item 33415Not much thought here, just went for whatever I could get with what mats I had while prioritising Affinity and Element Up
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Now this next one, I've jigged a couple of decos around to get Ice Attack to 6, in exchange for Divine Protection and Flinch Free.
Skills
View media item 33416View media item 33417Decos
View media item 33418Weapon Custom Upgrades
View media item 33420Again, just went for whatever I could get with what mats I had while prioritising Affinity and Element Up.
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That first set is what I got my 5 or so wins with. I've tried a Dagrix inspired Safi build and, while very powerful at getting the elem DPS check, I just couldn't deal with the need to sharpen (running around waiting for an opening cost me DPS and the win). Maybe I need to just keep attacking despite sharpness/git gud? Dunno, I'll give it a crack again but Master's Touch is great here.
First win took a Health Gadget revive, and 3faints with Safeguard. Out of the ~5 wins, I've only ever beaten him once with no faints (no safeguard, but did use Palico's Health Gadget revive) - I believe I got it before the third or fourth nova. The last two I've cleared with one faint before the fourth nova (both needed an additional Health Gadget revive).
As the guide shows and, as you probably already know, the aim is to get at least one elemental check in his first element, then the first horn break once Ala switches to Dragon.
Escaton. Mega potion/Astera Jerky/whatever.
Now if you've got the horn break, Ala should start in its first element again, albeit more aggressive. Aim for the second elemental check, followed by the second horn break when Ala goes Dragon.
Escaton. Mega potion/Astera Jerky/whatever.
Again go for the third check and then beat the ever living shit out of it as best you can. Head has the largest raw hitzone - aim for it as much as possible.
If Ala makes it past the third Escaton?
Your element isn't going to work in this round until it switches to Dragon. It's still possible to get the check but your game has to be flawless.
If it's not dead by this time and you've fainted to Escaton, hopefully you still have two faints left. You should be able to take it in this round. If not, try again.
I hope this helps. It's a fucking hard fight and took me ages. I love it. Arguably shitty mechanic but I love this fight. Good luck to you.
And happy hunting.
tl;dr: use the very first link