His Tentacles have 3.800 HP on Gamma, 8.500 on Beta and 15.250 on Alpha. So if you want to be able to destroy them with one clip you need a pump-action shotgun with at least 581.3% Damage and about 300 shots on Gamma, 400-450 on Beta and about 500-550 on Alpha if you use the shotgun exclusively for the destruction of the tentacles.

I also recommend a fast hun such as a pistol to destroy the homing balls that come for you, the damage is irrelevant for them.

Cactus Broth is also very good as it makes all creatures completely ignore you unless you attack them or get too close.