Depends what you like doing, If you completely fail at fighting get into a carat farming NHAA room.
If you are pretty good, get into a good team and play KOTH or Caged Beast, you'll get 120-150 carats every 3 round game.
Sliver british's 90 days are great if you have the astros.
But theres also a fun "glitch" i found, not sure if they fixed it yet, but you can keep doing Tutorial and sell the clothes you get from it. Yeah its really boring, but if you have a boy and a girl character, you can get 300 carats every 2-3 minutes.