Iron Citron also specialises in anti-mech warfare. The Mech Breaker will actually damage the Imp inside, provided that you hit the *pit. It is on the wiki, but I didn't trust the wiki until I killed a fresh Z-Mech with three shots.
Are you sure? I tryed over and over with a friend in the backyard and nothing happened.. I thought wikia loved trolling XD
In the backyard, you get a 90 percent damage resistance once your health is depleted, but I've done it in multiplayer on several occasions (or there was somebody else already damaging the Mech without me noticing it, but I doubt that.
