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.