To unlock this achievement you must get the stealth power up. The stealth power up is the blue stick figure. Fortunately this power has a high chance of appearing from the item box guy who appears after defeating a rainy enemy (20 kills to change to rain). Once you get a hold of this power, make a save. Use the power and then get behind the clueless enemies and grab them with . Each grab should be an instant kill. You’ll likely get around 7 kills with this before the power runs out, so reload and get the last 3 kills. The power up should look like this in the bottom left corner:
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