Majin and the Forsaken Kingdom

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.

I found a better solution if you can't use the "rock farm". In the next video you can get the achievement without restart your gamesave and unlock it when you want. The video isn't mine, video guide credits to Tripy73 of YouTube.

First i can confirm that killing enemies with explosive barrels count as a trap kill.

There is also an oppurtunity to get that achievement later in game. This is faster than restarting the game.
Just go to Potrachi-Altar(I think that's how it's spelled). There are two enemies which can be killed by explosive barrels (Thats the area where you have to use the majin to reach the ladder).
It takes 2, sometimes 3 of those barrels to kill one of them.

After you killed both of them simply go back to the Save Point and these enemies will be back.
If you got 20 trap kills earlier in game it will take no longer than 10min to knock of that achievement.

Additional helpful information is that even when you have not defeated 30 enemies by using Traps that can the Majin trigger, there is a way to accumulate the remaining progression about this Achievement: Explosive Mines are also Traps. Damage the enemy enough, then aim with LB to shoot an Explosive Mine when the sufficiently enough damaged Darkness Enemy is beneath it. Perhaps Explosive Barrels will help as well - but this is only believed (I give it a 51% chance).

To make this video game reload area status, perform this: Go to a Save Station, select Change Outfit, after one second press B.

In order to earn this achievement, you have to defeat your enemies with traps at least thirty times.

Pele Bridge is the best place to earn this one. At the beginning of the level, have your Majin wait and go down the ledges on the right. Keep going to your right and you'll see two red rocks near each other, but at a distance from left to right. You will find some monsters here as well, those being one monkey, two shielded warriors, and finally the one-armed tentacle monster. That's four monsters in total!

Go ahead and confront the four monsters and make sure the monkey and the two shielded warriors are following you. Stand by at one of the red rocks and when all three monsters surround both you and the red rock, go ahead and throw a rock into it. It will explode, causing you pain, but it will also cause them pain as well. It won't kill them instantly, so you will have to wait until they are back on their feet. If the one-armed tentacle monster is annoying you, just do a lunge attack with your melee weapon and he fall for a short time. If he's back up before the others are, do it again. That should give you enough time for them to get back on their feet. When that happens, lead them to the other rock, get them to surround you and the second red rock, then throw your own into it. This time, it should kill all three.

Then run away and go back towards the Majin, who will instantly heal your wounds if you stand patiently and if he is in Wait mode. Then go to the save point, pretend like you're going to change your costume, then back out of it and the enemies that you killed will respawn. Go back and do the method that I accurately described again and again until the achievement unlocks.

Once the 30th enemy has been killed with a trap, the achievement "Trap Master" should unlock for all eternity!

Have fun and good luck! Also, check out the video that Necophagus showed in his solution; that shows you exactly what I speak of!