Main Menu > Fight > AI Battle
If you're new here, you'll have to pick 3 Defenders. Any will do.
Now, select Find A Battle. From here, pick your opponent & 3 Attackers.
Then, start the match. Press the Select Button twice to speed it up by 4X.
You'll unlock this in no time. You can also unlock all Fatality Achievements & the "Blood In The Water" Achievement by following this method. Just be sure to select those that you need the Fatalities for. Hope it helps!