You cleared Offline Mode Time Attack within 20 minutes.

For this achievement you need to clear Time Attack in under 20 minutes. Angel is your best bet because of her lazer. To perform it hold X + Y and it will shoot out of the jewel on her forehead (Or you can just map LP+RP to your button for a one button lazer). I personally did it in under 10 minutes with the help of Angel.

My method with Angel: She's basically there to make space. With your other character do a small combo to knock the CPU back and then tag in Angel. Hit them with a lazer and back up. When they get back up off the ground they are likely to crouch for a second or two before getting back into standing position and trying to rush you again (only to get hit with a lazer again). If there is enough space you can shoot the lazer and they'll duck under to try to rush you but you'll have enough time to do a second one that will hit them.

Some characters such as Xiaoyu, Wang, and Ganryu crouch a little when they walk making Angel's lazer just barely miss them if they are a certain distance from you. Try comboing these characters with your other character to make them (hopefully) switch out with another character that doesn't do this so you can switch back to Angel.