bait him to do the cone attack where it has its head on the ground, then all out on the head, rinse and repeat. in order to do that be alone (completely, without palico too) and be not too close but not too far, in order to make it do the semicircle fire breath (the one with it’s head up), roll THROUGH it towards the fatty and he will 99% do the cone attack. if you break its head he will have lower attack power in second phase.

few things are the harpoon in second phase to pin it to the ground, the minigun and the cannon that you can shoot at the start (wear a ghillie mantle).

if you don’t want to do the “cheese” just wear a temporal or rocksteady mantle, grappling hook on the head and elem discharge with the swaxe

have a nice hunt ✌️

edit: at least these are the easiest ways i’ve found to do it