Manghost

Manghosts can possess zombies to make the explode or defeat them instantly.

Strategy
Manghosts can possess multiple zombie, so here is how he possesses zombies: 5 normal zombies, then 3 armored zombies, and 1 zombie stronger than a Buckethead.

Final Waves are where he gets useful, if you  have the Instant-Recharge Plant Food upgrade, and you have TONS of plant food, try planting loads of them. That's all.

Trivia

 * Manghost is slightly transparent.
 * Manghost is a portmanteau of mango and ghost.