I had the same problem initially with Daredevil, then realised I had to switch tactics after breaking both bars and just brawl it out. To do that I ended up taking Staff + S/P, Infilitrator's Signet, Roll for Initiative, Bandit's Defense and Impact Strike. After dodging and vaulting through the first phase using staff, I'd Steal to Turai and use Impact Strike to break his bar. Impact Strike wouldn't recharge in time for the second breakbar, so after Stealing I'd swap to S/P, use three Headshots and autoattack until he recovered. Then after that it was down to spamming Pistol Whip, along with Bandit's Defense and Impact Strike to cover me when I was low on initiative.