Apparently the Instant Kill Goomba works on: Pyro, Scout and Spy and theres a way to replicate it:
1) Spy: When dead-ringered, pull out the dead ringer as soon as you stomp the Boss.
2) Scout: (Unconfirmed) After the Bonk Atomic Punch expires, you can kill the Boss too.
3) Pyro: With Thermal...