Predator technology enable them to fight aliens with advance alloy which resist acid and/or acid proof.
AVP demonstrated it. One thing annoy me though, Anderson inconsistence with predator weapons that are acid and non acid proof. I believe it's bullshit. You have either the tools which are resistant or not. The concept or the idea to bring both of them on screen is ridiculous. So when you go hunting, you bring the adequate tools to kill you prey thus your are a professionnal. If your are a professionnal you do your job right not half of it. Anderson play half of it imo.
http://www.picsoup.com/images/140593 Aliens killed.JPGhttp://www.picsoup.com/images/77004Wristblade.JPG