From Group shop, redeem Medium refines (i assume you're f2p?, so those would be valuable; otherwise Kisame or wait for konan)

Edit: Karin is farmable from Elite instances and Survival, so she is a waste of group points. She is good, just... not from group shop. Those refines stack up over time. This post was last edited by Mitrandir at 2017-3-31 18:45

She can drop as a reward from clearing survival trial.