Majin and the Forsaken Kingdom

If like me you didn't get this cheevo by the time you reach the castle, you can get this in Arkela Castle - Hall.

Jump on the Majin's back and the hold onto the ledge of the pillar with the archer on it. Wait until it ignores you, jump up onto the pillar (while holding the crouch button) and press 'b'. You should be able to assassinate both archers without aggro'ing them.

Once you've killed both of them, go back to the save point just down the hall, change costume and they'll have respawned.

Took me little to no time to do it this way and you can also do the other grind-fest cheevos in the same room directly after you've done the achievement. Just make sure you don't complete the room otherwise the archers won't respawn.

This one is easily obtained through normal game-play but if you really need it you can spam the eight dark soldiers that are guarding the first scarecrow mask early on and also afterward in the graveyard there is six more dark soldiers. Just get to the save point and change costumes, rinse and repeat until it pops.

In order to earn this achievement, you need to destroy fifty enemies with sneak attacks.

You can earn this achievement as you are playing the game. If not, Arkela Castle - Hall is your best bet for this.

All you have to do is hold down the crouch button, sneak up behind them, then press "B" when prompted. If done correctly, your hero will kill them from behind. This is best done on either archers or enemies that are or are not carrying shields. You can find them all over the game's landscape.

Kill fifty of them and the achievement should unlock after killing your 50th enemy. Have fun and good luck!

It is good to note that you don't have to slowly sneak up to enemies when sneak attacking. Just run up behind when their back is turned, then quickly crouch with L and hit B when the prompt appears before they turn red.

I missed the chance to do near the beginning of the game by the first mask, but I found a good farming area a bit later on (I was about 4 hours in, but you can likely get there a lot quicker if you aren't worrying about waiting for memory shards). A few areas after the first boss, shortly before getting the lightning fruit, you will come across an area named "The Royal Garden" on the map. It is the area just past the one with the catapult and water wheels.

When you first get to this area, there are four of the shield type enemies. You can get three stealth kills very quickly if you learn their patterns. Start on the high ledge near the entrance and drop down behind the one in the corridor next to the buckets you fill with water to get the glowing blue fruit. Quickly hold L and then B when you are close enough. Head back toward the entrance, and two of them should be just heading back around the square pillar. Run in behind them, and quickly hit L and then B to get one and then the other. There is another one further in, but you will have to wait a little bit for him to get into a good position to be sneak attacked. If any of them spot you, just quickly run up the ramp near the entrance. They don't follow you there and will lose sight so you can quickly jump down to sneak attack them as they return to their positions.

When you have done that, return to the save point, select "change costume", exit out, and the enemies will be back again. Also don't forget to have the Majin "wait" again, because it resets his commands. After about 10-13 times (or less depending on how many sneak attacks you had from earlier), the achievement will pop.