I agree with the general notion of stronger single-target attacks. For the Overmind I would just have it target a single enemy at once. It actually aready does this with its eye and doing something to only that human would be trivial to implement. Your reactor idea sounds pretty solid to me.
kharnov wrote:As for the overmind, how about having it distort your vision and disrupt your audio?
Not advisable in a competitive multiplayer game. It will be cheated away and if it's not supposed to completely blind you (like a flashbang or smoke grenade in Counterstrike) then this kind of cheating is undetectable. A color grading and sound which is not meant to disable you seems fitting though.