The aliens really need to work on terrestrial transport that is invincible to combustion gunpowder weapons. If they have discovered force shields then I'm sure this would be solved easily, but then the weak part of the system would be powering the system. I assume the aliens have armor made of exotic metals that must be super strong and light that far surpass the armor of any depleted uranium or rolled steel we have.
Or the aliens need to learn tactics to protect their always vulnerable "weak spots" so that the humans don't capitalize on this. They must have high casualty rates because the the aliens don't seem so enthused about going on the attack. They must encounter more easily conquerable planets where there aren't man-apes with guns. Perhaps they just grow new soldiers but they always end up poor soldiers. So the movies teach us, alien armies are weak in morale and most likely made up of conscripts of a lesser class within the alien civilization's social hierarchy, who could care less about expanding their empire for wealthy prick-alien-bureaucrats who are always warmongering because they are in control of the alien defense industries.