This boss, despite his appearance is rather easy.

He'll start fight on ground with the Sword of Aeons. Block and roll under his attacks to get close to him. Hit him and he'll block, so work up a flourish and take his health down. At half health, he'll grow tall and float. Now, go beyond the stone pillars thar rose up, away from Jack. Shoot arrows to lower his health. Jack can summon powerful minions, so lock on and keep stepping to the side. You should avoid the blasts and for a while, the minions, too. When the minions get close to you use Force Push and continue shooting him. Here's where being behind the stones comes in handy. Jack has a move in which it constantly takes health at an unheard of rate. Hide behind the stones to avoid that damage, but now you must fight minions. Continue shooting him, and eventually he'll die, leaving you with a final choice.