If this is about their very acidic blood and saliva: been a long time since I took inorganic chemistry but you might get a pretty good reaction with their blood if you shot them with a sodium bicarbonate bullet, but also shooting then does work anyway?
But it's not just 'very acidic'. It's insanely, outrageously acidic. Anything that corrosive, acid or not, doesn't just eat it burns and explodes. If their saliva is that bad you can set up an ammonia cannister, turn it on, and listen to the thumps as every alien in the building's head explodes.
I was semi-serious about this? I was wondering what weaknesses such an extreme organism surely has, and chemicals as corrosive as xenomorph blood explode on contact with common things like... water. Certainly anything remotely basic. A xenomorph is a walking bomb. I bet ammonia gas would do it.
What if it's more like enzymes that go berserk when removed from the body? Like super evil clotting factors. Do we know it's acid besides its effect on the outside world?
They call it an acid. It doesn't matter very much. If it's that corrosive, any tiny nick in the armor makes it vulnerable to blowing up, and if it does it will open up any nearby xenomorph and they will blow up from contact with the contaminants.