No, you can't win credits by fighting people....you can win battle points and clintz that way.
You can win 1 credit for each time you end up in the first half of the chart after each tournament (3 tournaments a day => max 3 credits without paying). You also win clintz for playing during tournament-time, usualy around 50 clintz....
Recruit people and then have them buy credits. you will get 1 credit for every 10 they buy
The only way to get credits without buying them is to play in the tournaments. If you finish in the top half of the players you get 1 credit.
I lied there's one other way. If you get friends to play, and you sponor them, if they buy credits then you get credits as well.