Alright so Heres a good tip.

Noooo u dont need mutated gigas or rexes, u just need a velonasaur(from the fire wyvern island), crank up to abt 600%+ dmg and use a grapple to get on one of the ledges and then just shoot him from the cliff, he cant get to u, therefor keep some health stuff like blood or brew, a few stones will hit u for small damage.