Those brain suckers are the reason why you should always carry a grenade on you. I myself always team my squaddies in pair in minimum for the same reason, so if someone get attacked from behind and he have one of those aliens on the head his teamate can shoot it for him.
If this fail or he can't do it for any reason, just take a grenade from the guy who is being brain sucked, prime it to 0 second and drop it. Your squaddie might be slightly bruised (having a full hp armor help not to) but the Brain Sucker will die from the explosion.
Note that the human weapon which fire really fast can kill one of those Suckers in one shot, and it is a very good reason to have some of those guns equipped in the early game. And it also train your rookies aim a
lot in the process...
What I usually do is give those kind of usi to the rookies with poor aim, those with average aim I give them the big guns and those with incredible aim I give them the sniper gun. Then as they get better I simply drop the usi guns, since they tend to become obselete quite fast anyways. Fortunatly, since they train accuracy so much I still use them sometime, until the Aliens bring some personnal force fields to protect themselves. Then they really become useless, and you should have very highly trained squaddies by then who should be able to kill a fly at 50 metters easily anyways.
