If you don't feel like using the imp or just want to get this done quick then listen up.
What I did was raise the flag in the backyard battleground and turned into the mech.
I then waited till I got a bunch of plants then stomped, repeat this till the mech blows up then run back into the zombie base to wait on your cool down then repeat!
Note: Only the reg imp can stomp, So make sure you are using him and not any other!
Hope this helped!