Ah the man has lost it completely. Despite the knife he is pretty strong and if you're not careful it may prove to be difficult. Also just keep attacking him do not waste your MP you will need it later! Jack can use Paralysis so have everyone equipped with a Zodiac Bracelet, which will be useful for later as well. Also watch out for his Slash attack it can take quite a lot of damage away. Other than that and him using Thera Seeds this shouldn't take long.