There are quite a few ways to get free credits.
1) Play in an ELO tournament (at the end of the week if you have done at least 5 fights and have a score above 1000 you will get credits).
2) Get the highest survivor score for the day/week/month
3) Watch the videos on the homepage (when you go to the homepage a pop up box should appear asking you if you want to, though it doesn’t always appear).
4) Get a high position in a DT.
5) Complete one of the special offers in the special offers tab on the home page.