[Cosmic Tortoise]

I agree. Manhacks are invincible unless you are using a shotgun

No, you aim at the top of the model with a pistol and it kills them in 1 hit.  You're aiming too low.

If you're going to replace manhacks, add Detective Manhack to detective shop, which is a remote control version of a single manhack.

Also replace manhacks with a different version that isn't slow to deploy.  There is a bee grenade version, but they attack everyone equally.