Not a comprehensive or good (in many ways), but it may help you to beat it. Please note that this guide is seen from the eyes of a casual player (and very subjective at that).
Guide to Beat Alatreon
Do Some Research
I know some of you guys want to rush through contents, or whatever reason. You can *any weapon elem/stat goes bonk* on any enemies. BUT, please do some research first, especially for Alatreon. Go watch some youtube videos, look at its attack pattern, read some other comprehensive guide, etc.
Alatreon has 3 quest basically. The first is the special assignment [Blazing Black Twilight], then the 2 event quest, [The Evening Star] and [Dawn of the Death Star]. For both [Blazing Black Twilight] and [The Evening Star] Alatreon will start with fire element, and [Dawn of the Death Star] Alatreon will start with ice element.
Alatreon has 2 stars weakness of fire for ice start and the opposite. Both form has 1 star for dragon. I don't think I'll have to explain, but the more star it has, the higher the elemental damage is given when attacking.
Elemental Weapons Please
Many players (unknowingly or knowingly) still bring status weapons (e.g. lightbreak weapons). This is a bad idea, for you or other players. Well, yes you do contribute some damage, but you don't contribute to the element break it needs to do the fight properly.
Some people (maybe) already defeat it using non elemental if you remember or see those "safi snipecannon button" meme, but if you want to beat it, if you don't have those meta weapons, use any high elemental weapons, especially for melee players. Any weapon that's on the far right on the tree would do.
Only 3 elements used in Alatreon fight:
Weapons from Legiana/(Frostfang)Barioth/Velkhana/Kjarr usually used for ice elements.
For fire, (probably) Rathalos/Glavenus/Kjarr weapons.
As for dragon... Maybe Shara (don't forget Free Elem deco/charm)/Xeno/Stygian Zinogre. Alatreon weapons are the most used weapons for dragon element.
You can also use Safi's weapons, but you need those element augments and upgrades (which most people build it for more damage), so I don't really recommend em.
Though I said to bring elemental weapons, to all Dual Blades player, be happy since you have [Wyvern Strife] and [Blizzard and Blaze]. Those weapons bare both element and status. [Wyvern Strife] has both fire and poison, while [Blizzard and Blaze] has both ice and blast].
You done your research yet? If you do, good, if you don't... Well, I'll try my best to explain its mechanic.
Basically you have 3 stages: Element Active, Dragon Active, and the most devastating ability Escaton Judgement (gonna call it EJ).
When it's in its element active, fire/ice will do good elemental damages while the same element as the active won't do any damage. After a while, it'll change to Dragon Active, you can use any element to deal element damage to Alatreon. Keep hitting Alatreon while it's still in this 2 stages untill you get the, what I usually call," element break", which means you deal enough elemental damage to Alatreon so you can survive EJ. This can happen multiple times. Our beloved Handler will give a voice line when it happpens, and each time you do element break EJ's damage will decrease, increasing the probability of you surviving the attack. Simply saying, no element breameans your Doom.
Every attack Alatreon does inflict any blight based on the element when it's attacking. Either bring those sweet, sweet, holy [Nullberies] and build your character for Coalescence, or use Blight Resistance deco/charm for safer playstyle (no additional damage from Coalescence tho).
Break its horn for, of course the material, and keeping Alatreon on the Element Active it start with. For example, say the starting element is fire, then it goes into Dragon Active stage, you do element break, but you haven't break any horn, it'll go EJ then changes element to ice, making the ice weapon you bring uselsess, and making it riskier since you must do element break only at its Dragon Active stage. Horn can only be broken when its Dragon Active stage. So... Don't forget Partbreaker as well. Search for 4 big spike-shaped-wall and floor on the area. Wall/floorbang it when it's in Dragon Active stage to help break the horn. Floorbang only when its flying, easier to bang but riskier to do.
About the EJ, if you manage to do element break, you can survive. Bring health booster+ (if possible) and use astera jerky to negate the extreme damage it does. Rarely, you can survive EJ without element break (I did it only once, the first time I did the special assignment). Very rare case indeed.
Items, Equipments, and Skills
Well, here goes... Every player has their own preferences. Can't blame them for not bringing this and that. Though, I can recommend these items and skill to fight against Alatreon.
- High potions.
- Max potions.
- Ancient potions.
- Buffing items (like demondrug/adamant seed, etc).
- Mega nutrients.
Basically any healling and buffing items, self explanatory. You can also bring flashpods, but pay attention to its anger. Some player likes to turn off the "eye-con", I don't. The "eye-con" will tell you whether it's enraged or not. Yellow means it's not enraged, red means enraged (pretty self explanatory, I think). Only use flashpods when it's not enraged, otherwise it's not gonna blind it.
- Any elemental weapon you confident using.
- Any mantle.
- Health boost+.
Use any weaponn you're comfortable with... Do not use status weapon. Self explanatory. Since I try every weapons and use the most comfortable with the enemy, I go with Dual Blades. Quick, multiple, rapid attack is easy to use for it IMO. Health boost+ helps you a lot to survive EJ. Use any mantle you're comfortable with, but beware for temporal/rocksteady mantle. Alatreon's forward-dashing-while-flying attack hit multiple times. Say goodbye to your health/mantle if you're not careful.
- Elemental boosting skill.
- Blight resistance/coalescence.
- Free meal.
- Weakness exploit.
- Crit eye/boost etc.
Some of the skills were being explained on the previous section, go read it again if you forget. Elemental boosting skill is important to increase your weapon's elemental damage. If you're like me (like to play safely, getting hit a lot) free meal can help so you have a chance to mot use your items, giving you multiple uses of 1 potion. You can use other skills as well if you want, just make sure you're comfortable with it.
- Kulve MR.
- Silver Rathalos etc.
Teostra'a master's touch is a good one to go offensive playstyle, making you sharpen less. Velkhana's 2 piecea is good for element damage boosting skill (crit element) and 4 piece is good if you're using Greatsword (frostcraft). Silver Rathalos is better for element damage since it's true crit element (superior than velkhana) and slinger capacity help you to have more slinger ammo so you can do 2x wall/floorbang with just picking stone 1x. Kulve MR skills, guts (2 pieces) and free meal secret (4 pkeces) can give you better survivability. As usual, you can use any other armor as long as you're comfortable with it.
- Crit element.
Well, this is pretty much Kjarr weapon's specific trait... Only this, I guess.
Be Patient, Be Comfortable, Familiarize Yourself
If you can beat it by just watching some videos, good for you. Otherwise, do it slowly. Fight it solo or with party, make yourself comfortable familiarize yourself with its attack, roar, etc. Failure makes you stronger than before. Practice makes perfect. Don't give up.
It takes me more than 30 tries on multiplayer, lots of cart, with many players left in the middle, making it either failed, or often times win. And I tasted my first solo win after so long. The adrenaline rush.