This is the best, and easiest way to kill jack:

First wat you need is

Solus greatsword
Sskorms bow (or master longbow)
Max out multi arrow and multi stike

Then go into the fight with jack, kill his minions, the put multi strike on and attack jack a few times, when he goes up into the air go behind a rock a put multi arrow on, if you have skorm bow you will only need 2 shoots him once or twice, and master bow 2 or 3 times. this will kill him very fast, and u wont get hurt very much. Happy killing!