When I started using more than just Thrash/Maul is when I became a much better Assassin. By the time I reached Zash, I one shot her without my HP falling below 50%. It was just a matter of interrupting her casts (especially heals), using my ranged dps, setting up for burst damage and using line of sight. I'm really enjoying the degree of intricacy the battle concepts offer. The only change I would offer is to bring back auto-attack instead of having to press a button to do it. My action bars are already crowded enough.