I think there are some things about the trials that do need fixing like in Harmless Telly Tubby's post, but I don't think the final stage necessarily needs fixing. I have found that if you select the archer class you can lure Zulrah to the outside ring and let him follow you around it while you shoot arrows at him. But, still be sure to keep your distance since he can do quite a lot of damage.
Good luck with trials!