I played for a little bit and I got to the 4th level, played it once and those little guys that 'multiply' totally owned me. Is there anything special you gotta do to kill those dudes?
Edit: On the 5th level now, beat the 4th with no leaks at all
beat 5th and now on 6th .. havent started 6 ... oh ya just to warn u .. end of 5th is hard .. 2 giant helicopters .. and 2 small slugs .. very very hard to beat them ... without 4 turrets for the helicopters .. and for the slugs .. i had 4 maxed out machine guns, 2 maxed out SAM's, 1 maxed out freeze, 1 maxed out shock, 1 maxed out Air turret, and a maze. I still had a hard time beating them ..