Have been trying to do the rock method and when Zorn hits 65% the berzerk on Toth breaks. Is this just us failing or some kind of bug ? Cause we have both tanks on toth switching out when one of them get the yellow circle. Any feedback or advice is appreciated.

You are trying to exploit an unintentional loophole in the game mechanic so it may not work correctly 100% of the time. I have been in groups that have tried this and could not get it to work. Personally I think people should learn the mechanics of the fights and complete them accordingly. It will improve your skills and will be far more rewarding. That is my advice and recommendation.

How do you know what he is trying to do is an unintentional loophole? From my viewpoint, not putting the purple circle under Toth, thus keeping him Berzerk is the game mechanic functioning as normal......he's just attempting a different strategy.