I'm a level 41 guardian but I went the DPS route. I've tried with all companions but the closest I got to beating her was with Keira. whats worked best for me is using my stuns the only problem is when the 3rd wave of reinforcements come they kill keira and then hareth kills me when i have her down to about one more hit. unfortunately this is while using a strength stim so idk what to do. any tips?