Everyone says "Level Zero" is best for this achievement, but I say that they're all wrong. The easiest level by far for this achievement is Level 9, "Howling Moon". There's only about 12 enemies in the whole level.
As you start the level, you'll swing from tree to tree and then when you first get down to ground level, quickly perform the Panda Quake move to take out the 3 wolves. Then take out the next 2 wolves (the ones that cut down the tree) with another Panda Quake.
Continue through the trees, being careful not to get hit by one of the swinging spiky logs until you reach your first sleeping wolf. Creep up to him by just pushing your left stick forwards very slightly, then hit Y to kill him in one hit. Go over the tree trunk you must knock down and kill the next 2 sleeping wolves in the same way (or perform a Panda Quake to take them both out at the same time). As you swing through the trees you'll come across a few more sleeping wolves. Creep up and kill them with the Y button.
Now, when it comes to the branch that has the swinging spiky log before it, you must time your jump perfectly in order to not get struck. I found the best way to do this was to hang from the branch until the spiky log passes you. The very split second that it does, press down, then up and then at the peak of your forward swing, let go.
When it comes to the bosses - the Wu Sisters - the easiest way to kill them is to block and Panda Quake. As soon as the battle starts, let them get a little closer (but not too close) and then perform a Panda Quake. Then run. Keep away from them while they're spinning. If they get too close, hold LT to block (it helps to have fully upgraded block). When they get dizzy, Panda Quake them again. Then it's just a case of rinse and repeat until they're defeated.
Much easier than "Level Zero". No tricky traps to navigate and 90% of the enemies will be killed without even putting up a fight.