The Z-Mechs are supposed to be formidable, not unstoppable. With the largest stack of Health of anything playable right now and the ability for some of them to dish out large quantities of damage it's no surprise that the minute that one appears on the battlefield it either gets targeted by Gatling Peas, SunBeams, or just plain ganged up on by the Plants.
Since a lot of Imp players tend to simply charge around with impunity while in their Z-Mechs it's no surprise that they get taken out quickly; they don't need any new buffs- just because you can jump in a walking tank and spam bullets and missiles everywhere doesn't mean that you can abandon tactics and if you're dumb enough to just wade into a group of Plant players, as opposed to a bunch of AI, thinking that you're unstoppable suddenly you get what you deserve.
If you want to play Big Bad Z-Mech devote time to Graveyard Ops and you can stomp on all the Weeds and AI controlled Plant Heroes you want while they flail around not being able to handle you but don't expect actual experienced Players to be as hapless.