My favorite way to get credits is to find a few friends and play friendly capture the flag. Once you get in a groove, it's very quick, and if you make sure that both teams always pick up their flags before someone hits the target, even the losing team gets a decent payout. If you and a friend can run two characters at once, you can each have one player that always wins and one that always loses, instead of having to take turns, which speeds things up even more - you spend less time thinking about whose turn it is to win.