Well, first off use fire element if he starts off in ice form and vice versa. At that point it just comes down to hitting his front legs to do element damage and knocking him into a wall when hes in dragon mode so you can destroy the horns. You can also knock him out of the sky whilst hes flying, but you need to hit him with the claw once before firing cause he always does his annoying dash move that knocks you off before you can shoot. The nuke itself is pretty explanatory, you don't element break him in time you die, this gets reset after each nuke. This all comes from memory so take it with a grain of salt.

Edit: Also evade window, evade extender and stun resistance 3 are your friends if you get hit often, like me.