The two game types i usually play for credits are Firefight arcade (when im desperate)
and grifball.

Firefight arcade is probably the best one to get credits, I usually get at least 1,000 credits. You have infinite lives so you can go into the open as much as you want (Your deaths dont affect how much credits you get) just try kill as much as you can with any weapon you got.

Grifball CAN get you a lot of crdits it depends on the players you play with. When you get the bomb dont try detonate it right away. When you have the bomb you defence slightly increases. It sometimes takes 2 hits from an energy sword or even 1 hit. But one melee from you is a instant kill. The gravity hammer is at least 2 hits (unless they hit you from behind) the least i ever got from a game of Grifball was about 700 cr.