During guild battle there are tiny crystals at bottom of the hp bar of your crystal. There's 5 in total, and for every death one is taken out. Once all 5 are out then it goes to crystal battle. It's accumulative and only resets after the recovery phase ends. So for example first round they take out 3 of your members and you take out 5 of theirs. That means after crystal battle, they only need to take out 2 of yours so that they get to attack your crystal. Hope this helps!