How to kill a manticore

If the sacred twins were invincible except to each other, what killed the other one? Just curious. You'd think someone might at least have wondered, as they looked for clues as to how to kill this one, especially since the evil guy kept repeating that it couldn't be killed. I'm somewhat handicapped by having only seen half of this movie. Also I keep wondering why it is that soldiers in these man vs. the beast movies always seme determined that enough ammo pumped into anything will kill it, even if it's a supernatural entity of some kind, or has armor that obviously repels everything under the sun. Maybe during training they all sleep with a tape repeating under their pillow saying "Keep firing, keep firing...."

"God, I hate mummies!" -- Rick O'Connell, Matt Fletcher, Danny Freemont, etc.