No matter how strong aliens are they still feel pain and are susceptible to physical force, so a healthy strong marine wacking them with all their strength with a solid object like a gun that probably weighs a good few pounds is obviously going to knock them back and if they lose their balance, then they are going to fall. simple physics.
Now if the alien and the marine was in a hand to hand fight, then its would in the aliens favour by a certain percentage, the alien would obviously be physically stronger and faster, can hit harder and take punches and attacks easier but in terms of physical force, they can be staggered and so long as a marine doesn't wrestle i.e grabbing and getting too close to the alien (like Dillion and Parker did) they could possibly beat an alien, all they would have to do is use strong kicks, punches and evasive tactics to outmanoeuvre their attacks.
