First of all make a Created fighter, put him in the featherweight division and put all of his stats to zero.
Put the game on beginner, go to the featherweight divison and pick Jose Aldo. Aldo has some of the best leg kicks in the game. Pair him off against your CAF.
When the match starts hold the LT, hold the left stick toward the opponent and start pressing A or B.
You can also run toward the opponent and hold the left trigger, press A or B. This would land a more devastating leg kick.
When you TKO him with leg kicks, the achievement is yours. Good luck.