Well use your creatures if you can to distract him. If your this far in the game you must have the necromancy creature as well as your summon undead. The undead will not last long but will give you maybe 3 seconds of distraction.
Get in close and make him draw his sword, then you can back off and he supposedly does not switch back to his bow. I played as a ranger but for him I got in close and dual wielded him into mulch with the help of my necromantic creature and also I had a level 1 or 2 demon summoned. They will help greatly in boss or harder fights as they get in close and force the NPC to draw melee weapons or at least distract and you can just hang back and pluck away with your bow. That's how I worked my ranger character at least.
You can also use that hump in the bridge to your advantage as he just mainly stands in one spot or at least he did for me, maybe because my creatures had him pinned down. Wait till he shoots pop up from behind the hump and take a shot while he's reloading, duck back down and repeat if you want to go for a straight ranger kill on him.
As for skills I did not use any of the ranger skills as my bow was deadly enough with 3 enchantments and 2 charms. Time I'd look down and find the right key on my keyboard I could have the character dead already by regular bow shots.
I did put my ranger strength up to 5 or even more though as that adds significantly to your ranged damage if I remember correctly.
And with ranger and magic damage, if your bow has it, put some skill points into intelligence as that raises your magic damage. Dexterity as well for ranger that will up not only your ranged regular damage but your ranged resistance some as well.
Enchant your weapons up with the best damage enchantments you have available and you should be fine.
