I don't think the films depict the Alien as omnipotent. In Alien a good spear gun works, in Aliens the light armor piercing pulse rifles blow them to bits, in Alien3 there is no real weapons to do them harm, and in Alien Resurrection the weapons work fine on them. The creature comes across invincible if you let the fear of it prevail, and there are reasons to be afraid, if it gets its hands in you, you're done, one way or another.