>>719183204
Any army should have no real difficulty with xenos, barring the extenuating movie circumstances that cause the plots of each movie.
In 1 and 4, they're on space ships so the acid blood melting the hull is a lethal threat. Guns are unacceptable.
In 3, the inmates are unarmed.
In 2, the marines are fully equipped to fight them but underestimate the threat and give up 95% of their arsenal before the first engagement, because they're fighting inside nuclear reactor that could explode if they shot its cooling tanks accidentally.

Fighting xenos with any kind of pre-prep is actually not difficult at all, assuming you have a gun to shoot them with and no plot reasons preventing you from shooting them.