Get a bow and about 30-50 arrows depending on your marksman skill and a few paint brushes, you will only need about 5.
Then head into the arena and speak to the guy who lets you fight someone.
Get into a fight and as soon as it loads the arena battle press the B button as quick as you can just before the
opponent runs at you.
Then drop a paint brush, it should hover, so jump on it, ignore the opponent for a minute and keep dropping paint brushes, then jumping onto them, but be sure not to get knocked off.
Get to where he can't reach you with his attacks and pull out your bow and arrow and just keep firing your arrows at im.
When you kill him jump off the paint brushes and get your arrows.
You will gain some marksman bonus and some acrobatics bonus, plus get to a higher rank in the arena.