This trophy is a bit of a hassle. It sounds simple enough, but has been causing people problems. Simply put, you need to burn enemies using either the upgraded Rossmore or molotovs, followed immediately by freezing them with the Cryogun, and then poisoned with the secondary fire of the Mutator. The effects do not need to be done in this order however.
If you'd like to try an alternate method, load up New York, NY: Subway. You will come to a room with several Grims and 4 Ravagers. Ravagers have a large amount of health so they are sure not to die mid-punishment. So walk up to the Ravager, freeze him with the Cryogun, let him unfreeze a bit then hit him with the secondary fire of the mutator, and finally, molotov or shotgun fire until he is dead. This should count as 1 towards your trophy. Now, if you are out of ammo you can either use the Infinite Ammo cheat, or let the enemies kill you and try again.
Credit goes to T_l_M_B_0 & AndrewAnonymous at PS3 Trophy