This trophy will most likely unlock naturally as you play through the game. Littered all around Pandora you will find barrels that can be shot and they will explode. Each different colored barrel does elemental damage related to that color (Red cause Incendiary, Dark Blue causes Shock, etc). While each of these barrels has an elemental effect, all of them will eventually explode given enough damage or time.
What you want to do is lure an enemy near one of these barrels then shoot the barrel to kill the enemy. It’s alright if you take damage in the explosion as well as long as the kill comes from the explosion this trophy shall be yours. If the barrel explodes but the enemy is killed by the elemental damage after the fact this trophy will not unlock, it has to be the explosion that kills the enemy.
This is also one method for netting yourself the Florida Man trophy for downing yourself by grenade or explosion.