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You'll need to vanquish 20 Taken with what are basically environment kills; electrified fences, explosive canisters, etc. There are big red explosive canisters throughout the game, shoot them once and they'll explode. However, this achievement is easily gained at the end of Episode 4 during the stage battle at Anderson Farm. Many of the Taken will be killed by the fireworks at this point. If you do not happen to get the achievement, reload your checkpoint and continue the battle until it does.