You know what was sort of funny? XD As you can see on my screenshots, Jack's health is getting lower and lower, yet I never actually attacked him. Maybe hit him with my katana once or twice, but he blocked those (except once

He was already hacking away at me for 5 straight minutes and I accidentally hit my attack button: he didn't block this one XD I guess he didn't saw it coming lol)
Anyway, at one point he suddenly Assassin Rushed to the centre of the room, where the second stage of the fight starts and I was all like "... wha? *looks at his health bar* How did that get so low?!"
Apparantly he was constantly damaging himself while blasting at me from close range XD Not very smart, Jack
If you have aroud 250 á 300 (I had around 200) health potions, you can beat Jack without ever having to swing a sword at him, use your bow or use magic. Just keep as close to him as possible while gulping down potions and fighting off those damned minions.
It's fun to try out once XD