1) Go to practice mode.
2) Choose a character with an easy to perform Rage Art. (I used Katarina as hers is just [cn_A] + [cn_B]) Opponent doesn't matter.
3) Once practice has loaded, open the pause menu and navigate to the Other Settings tab.
4) Turn on Player Rage here.
5) Now unpause the game and activate you're rage art 20 times for the achievement!