Easy "Trap Master" Grind
This achievement had me worried when one of the first large traps glitched on me, close to the beginning of the game - where the large boulder was, there was no enemies; and finally, when enemies spawned in, the boulder was gone and I had not touched it! Thankfully there are some reusable traps and I've just come across an easy place to grind this achievement out so worry not!
It's very far in the game at the Crystal Palace - Interior. You'll see an L-shaped staircase with archers and those red things, and a handful of enemies around some pillars on the ground floor. Have the Majin wait or attack the enemies on the ground so he doesn't get in the way. Kill the archer's by attacking them a few times (as few as four hits will do it) then throw a rock at the explosive red thing on the ground. There are two archers you can do this to, then the save statue is right where you entered the room - just access the costume change screen and return, everything will respawn.