Use LBG Soulfire Rasp “Styx”. 1x recoil assist mod and the rest reload assist mod.
Equip these Skills: part breaker, blight proof, evade extender, health boost, gobbler, divine blessing
Pelico: Equip Plunder Blade, Tigrex Armor, Paralysis Weapon
Bring Slash Ammo and Sleep Ammo(II)
Bring Temporal Mantle, health items, etc. duh!!
When fight begins, immediately roll behind Alatreon and keep shooting its tail from behind. After 50 rounds of slash ammo its tail will break.
Now use sleep ammo to make it fall asleep and go carve the tail. After it’s done, wake up alatreon and die and fail quest on purpose. You should get other items from Pelico rewards.
If you understand all the above then you don’t have to waste time watching my stupid video.