You can earn free credits in UberStrike. It’s not truth that you can get credits only if you buy them.
You can earn credits participating in different events and competitions. Atm there are couple competitions where players can earn credits. Make sure you follow Announcements in Uberstike forum http://forum.uberstrike.com/viewforum.php?f=1 , UberStrike page on facebook http://www.facebook.com/uberstrike or follow Uberstike on Twitter https://twitter.com/UberStrike
Have fun earning creds :)