With Arc Burn it can be pretty tough, sure, especially because wizards deal arc damage as well. What me and a friend did was use the Titan bubble strategy, which involves popping a bubble right under Omnigul with Helm of Saint-14 so she becomes blind, and then keep a knight alive so he can shoot the bubble which will generate orbs of light (you need to have Gift of the Void though). Then just blast at her with your best arc Heavy weapon or punch her to death! The other person pops up his bubble after the other one is about to finish. Rinse repeat until she dies!