If you have enough controllers, 2 is enough for most, but 3 and sometimes 4 is better. You only need 4 for one achievement, but it makes it easier. The characters to use are:
Captain Planet, 3 simultaneous knockouts
Toiletnator, 2 simultaneous knockouts
Dexter, 2 (simultaneous) knockouts
Kevin, 4 simultaneous knockouts (lasts for a while)
Jack, 5 kills AFTER activating, just dont die.
Billy and Mandy, 4 kills AFTER activating, just dont die.
Scottsman, activate PTE and activate Peppermint Larrys assist. Easiest with 2 controllers.
If you play the final stage in Samurai Jacks story mode, you can get PTE achievements easier there against normal enemies.