Seeing how that total surprise attack didn't work at all (the monster is complete unharmed), it's safe to assume the monster is unbeatable. Unless tentacle-bro can somehow beat it, maybe from the inside out.
Uh, if they take out their sensitive part while penetrating a women isn't it reasonable that:
They make a weapon that's supposed to be inside a women's genitals(both front and back) so when they are really in a pinch they can just activate it and it blows up the insides of the tenticles.