When you encounter your first Legion Champion on the first island, be sure to have stocked up on throwing axes, get his attention, and throw two at him, if you are able to squeeze one more in there, do so. You will know he is weak by the his posture, when the button mashing finisher is near, he will slouch, he'll make a final strike and then you must activate the BM session.

*Note:* You may also use LT + Y to evade the attack if the "throwing axes," fail.