Well... when somebody said "so its basically a ranged attack which makes sense" this idea came to mind.
But anyways, perhaps this could be a building that moves, based for aliens, that will go in the direction of the human in its sensing range, and fire its lovely attack at them. Or it could be a mod that spawns AI neutral versions of those (Just making a name up here, dont say "blarghisketch! thats not what it should beh cawled!") Flubbers which attack anything they see. They spawn at, say, 30 min or so, in the middle of the map (or wherever the Flubberlord wishes it to be)