I feel like a lot of people underestimate how tough Xenomorphs are.

I see a lot of people toting the idea that "Xenomorphs are fast but they are fragile", but they miss an important detail I can't help but bring up that a lot of people miss is that the standard issue Pulse Rifle for the Colonial Marines uses 10x24mm Caseless EXPLOSIVE-Tipped ammunition. This is not only a big bullet but it also has explosives in it.

The amount of force of these rounds would have would be considered overkill, these rounds are both extremely rare, prohibited and are considered a war crime to use. Even then the Marines often get slaughtered despite this ripper of a round.

So in reality Xenomorphs are tough as hell, if that's the kind of ammunition you need to kill a xeno. So in both of the not canon AVP movies it was realistic that both the mercenaries and military forces were torn apart.