Awesome advice mates, so I'll offer my own as well.
Darkhart is a 2 hit kill for me. One +5 Siege (notice I before E ecxept after C...), with which I also have +5 long volley, also increasing the damage.... not to mention +5 barrage... strengthening them further, then fast switch to blunderbuss (the siege itself usually does around 10k) and finish him with a +5 Take Aim, +5 Silver Shot from the blunderbuss.
Doesn't get any easier than that.
Just my 2 cents.
*Disclaimer, using the broadsword combo is also a quick way to bring him down , especially if you don't want to go running into the Ammo shop all the time to buy more siege (again, I before E except after C... sorry for the grammar patrol, it's just a pet peeve, hehe).
See ya out there, guys.