We dont necessarily have to outright stun the aliens. Tear gas is non lethal, but it doesnt cause unconsciousness. So make the grenade launcher something that will cause the aliens to...I dunno....not do anything for a turn or two, or one turn with greatly lowered accuraccy for a few turns following, as well as lower moral. Something that will tactically aid troops with stun rods, or even to kill the aliens.
I understand that this would also make things easier. But its not like the aliens dont know about things getting in their eyes, right? Undoubtedly they would have something along the lines of gas-masks, if not so bulky and detrimental. A combo of gas-mask a night-vision goggles, perhaps?