Godzilla is the perfect example. Godzila can be beaten sometimes, but he can never be destroyed. That only pertains to the old Godzilla, though. The new one is not a monster, but an attempt to make him more realistic. For all intensive purposes, the new Godzilla is a dinosaur, and we all know how well dinosaurs survived.
Monsters are the only way to go.
They can adapt better in any environment, can smash thousands of robot droids before getting winded, and the toughest of them can withstand the force of atomic bombs.