I have never done this credit event (or keycard battle, as it was called before the change) since I always had enough credits.
Now, with more and more chars to lvl and star up, I could use a few extra credits. So I did this event for the very first time a few days ago. The description says "base reward 8400 credits". So I thought I maybe get this ammount for every opponent killed or at least for every battle I can complete. But no, I got 8400 credits after the first battle and thats it. LOL, 8400 credits for 24 energy? Srsly? That's 2858 energy to get 1 char from 6 to 7 stars