Ok, so for this one you need to kill 30 enemies with traps.
Traps are anything that you can instruct the Majin to push over and kill enemies, there are two walls in the opening area before you escape the castle, a couple places where you can roll boulders or push over pillars, etc...
It is possible to miss this one if you don't make full use of each trap, which is what happened to me.
What I ended up doing was starting a new game from scratch and rushing through the beginning, after you escape the castle and go through a couple sections of the map, you will get to the first area where they show you the "scarecrow" with a mask and tell you to break it to unlock the door, etc, etc...
This area also has a boulder you can push to get three kills. As you going up the hill, instruct the Majin to take position (I think he does this automatically the first time in a mini cutscene). Then once he is ready, go to the left to attract the three enemies. Retreat down the path and tell the Majin to roll the boulder once they are all in the path with you, and you'll come out of it with 3 kills. There is a little cubby hole in the right side wall as you are going down the ramp where you can jump into if you don't want to get hit, but the boulder shouldn't kill you even if you do get hit.
After this, go the rest of the way down the ramp, save, exit to the title screen, and re-load your save game. The boulder and enemies will both have respawned, and you can repeat this until the achievement pops.
If you got 9 or 10 kills in the opening area with the two walls, you'll still have to do this 7 times, but it goes pretty quick. Starting from the beginning and ignoring anything that isn't necessary, you should be able to get here and farm this out in about 45 minutes or less.