Elementalist
Had an enemy frozen, corroded, ignited, and electrocuted at the same time.

---

There are many ways to get this trophy, some easier than others, just remember that if you try to get all elements the hard way then you will need oxygen to light enemies on fire.

The easy way to do this is to get your Claptrap to level 25, which you will need to do anyway for his Action Skill trophy and unlock Rainbow coolant in the Fragmented Fragtrap tree. This ability will emit a Rainbow Nova Consisting of all 4 elements whenever he changes Sub-Routines. To get the rainbow coolant to hit someone with the coolant you will need to be standing next to an enemy when your stacks go from 99 to 0 (located above your XP bar). When you get a new subroutine is when he does his rainbow coolant special automatically.

Another method is to use Fragtrap to Duel a friend or a Splitscreen character, wait until your new sub-routine to come up and hit your Duelling partner with it and the trophy will unlock. You will still need Rainbow coolant unlocked and the opponent would need to be close to the level of your Fragtrap. Thanks to taliesin2943 for finding this method.