Get twenty Silver Keys and go to the silver chest at the guild to get the Legendary Greathammer. Get your Muscle stats very high. Get good Assassin Rush and Multiarrow, along with a good bow. When Jack stops floating at the start of the fight, Assassin Rush over to him and hit him hard. When he starts floating and doing spells on you, just multiarrow. You can defeat him in a matter of minutes. Note: It is not required, but Slow Time can also help.