Kill 2 enemies with one fireball.
Another easy way to get this, is in Mustacho's Dungeon (lvl. 6.1). In the room with the lift, where you have to stand on a platform to raise the lift, there will be enemies coming from both directions. The last 'wave' is two enemies from the left, shoot a fireball between them, and the achievement should pop.
This is also an easy spot to get
Just shoot the enemies, reload checkpoint, and you should soon have this.