In "Laboratory Retriever" you can stand on the platform in the corner and throw the bottle of goo at your opponent while easily dodging various items they throw at you. This way, the only way they can hurt you is to climb up to the ledge, and then you can knock them off to the side without taking much damage. Also, there's a weapon you can grab on that platform that knocks them off and does a great amount of damage as well.

Successfully complete the game with Tom or Jerry to unlock Duckling.
Successfully complete the game with Duckling to unlock Butch.
Successfully complete the game with Butch to unlock Spike Jr.
Successfully complete the game with Spike Jr. to unlock Tuffy.
Successfully complete the game with Tuffy to unlock Spike.

Successfully complete the game with Spike to unlock teamplay mode.