Go to Practice Mode and pick Glacius as your character. Pause the game and use either bumper button(RB or LB) to select Dummy options, then change Action to "CPU". Then bumper over to Practice options, go to Instinct Meter and right or left toggle to choose "Infinite". Leave everything else as is. Once you unpause the game hit HP+HK(RB+RT) at the same time and enter Instinct Mode. Once active, your armor will be on and the CPU will hit you and unlock the ach.