Go into options and set match time to 300 and life bars to 1 and 1 -round (so you both start on red health, making matches extremely short, but you get a big time bonus).How do you power up a character to 20? Sorry still Noob.
If you have a 2nd controller, just go into 1v1 against an empty opponent and get perfect after perfect (make sure Comp mode is OFF, you want gear and levels to affect your damage for faster wins.
If you don't have a 2nd controller, fight a level 1 AI on Very Easy and use as many unclashable moves as possible. I play against lvl1 Brainiac, since he has no great wakeups.
Takes about 30 minutes to go from 1-20 if you're efficient. So far, Red Hood is the easiest chaarcter I've leveled. Start each match with b3-b3 and then either zone or spam df3.