This requires that you kill 100 enemies via exploding a vase rather than hitting them. While there are plenty of vases throughout the game, if you need to grind this the first two levels are perhaps easiest because the enemies are weaker.
The best approach is to not hit the enemies at all. Merely gather some enemies around you and then go hide behind the nearest vase. The enemies will hit and ignite the vase trying to get to you, and the subsequent explosion will damage or kill them.
The achievement will pop after the 100th kill.