NINJA BLADE

Not too far from the beginning of mission 6 there is a long corridor with a white floor, when you arrive in the next area jump down to the ground where there are 2 pox giants, in this area go beyond halfway down and there will be a small group of bats, either use the dual knives on these bats or specific wind shruken move (hold any direction and tap B twice) and you'll get above a 10 kill combo. Now just progress through the rest of the level and the achievement will pop at the mission results screen.