Well, in avp, the aliens were stupidly conjured... so i don`t care what was fair and what wasn`t. I can`t agree with anyone who says "I don`t see what was wrong with the first movie" because the first AvP was utterly stupid. I posted most of the stupid stuff in the movie in some topic... Ofcourse aliens aren`t unstoppable, no living organism is unstoppable, but aliens aren`t stupid and don`t just act on instinct. They have the same Intelectual capacity as the humans and predators, maybe even a higher intelectual capacity than both combined, since the alien`s brain mass is much bigger compared to the rest of the body mass, which means that it has that much more brain of the brain to use for thinking than for controlling the body. This means that they learn very fast, and have good memory. But even so, they aren`t born with any knowledge so their intelectual capacity is not very important.
Now... why would an alien not be able to kill two predators since you`ve seen one do it? Some people say that in reality the predators would bla bla bla bullshit. That`s how the movie is and that`s the only reality you`ll get. A naked fella with no eyes who eats your brain is just as ustoppable as a dressed fella with heat vision who takes yor skull. They are just two different creatures, one vain and dumb, one shameless and dumb. Well, at least in AvP they were both dumb.
So anyway, if a predator can hold on to two aliens at a time while they do nothing, then those aliens have to be pretty dumb, but still, if you only have one predator, what do you expect? It always depends on who the main character of the species is, because that character will kill more than any others of his species.