-Get twenty Silver Keys and go to the silver chest at the guild to get the Legendary Greathammer. Get your Muscle stats rather high, then 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, simply multiarrow. You can defeat him in a matter of minutes. Note: It is not required, but Slow Time may also help.
-First, make sure you have the following spells: Physical Shield, Assassins Rush, Force Push or Enflame (recommended), Multi Arrow, and Berserk. You will also need to max out your health. Get the Master Longsword and put two Sharpening augmentations on it. They are sold in Knothole Glade. After that, purchase a Piercing augmentation from Oakvale. Next, get the Master Longbow. Get three Piercing augmentations and fill your bow with them. Next, buy as many mana and health potions as you can find. Max out all of your spells. This may take some time, but it is the easiest way without having to find Ages Of Skill, Might, or Will potions. Go to Oakvale and kill the guards. When asked for the fine, say "No". You should be able to kill the first guards easily, in about two hits. The red guards should also require about two hits as well. For the black guards, use the following combo. Hit, Flourish, then get behind him. When he gets up, hit him from behind. He should be finished. Do this until you get enough to max all your stats. Face Jack Of Blades. He will send out a wave of Old Kingdom rats, It should take about three hits each to finish them off. For the spell casting rats, use Assassins Rush so they do not hit you with any spells. When Jack turns the protective bubble off, use Berserk then Assassins Rush. When he hits you, you should not fall down. If you need a potion, use one. If your timing is perfect, he should not be able to hit you for six times. However, after about the sixth wave, he will spawn some rocks in the ground, then fly into the air. Get behind the rocks as fast as possible. He should spawn some rats. Do not worry about them; just use Physical Shield, then a Will potion, then Enflame or Force Push followed by Multi Arrow to get Jack of Blades. He should be down in about four or five. You will have to recharge your multi arrow only once.
-During his first form (human), use Slow Time and Enflame to kill his minions. By this point you should have at least Slow Time 3 and Enflame 3. Taking care of the minions by doing this should not be too much trouble. Enflame deals damage even when the minions are on the ground. After Jack stops floating and starts attacking with the sword, do not waste time Assassin Rushing or casting any spells. Run directly up and attack him. After he blocks a coupletimes your weapon should glow purple. Use your flourish attack and he will take damage. Your weapon should still be purple. Keep hitting him. If it is not still glowing purple after you hit him, just attack and let him block you. After you hit him two to four times with your flourish, he will Assassin Rush away from you and cast some magic. Evade it as best you can, but again run directly up to him and repeat the Flourish process. You should have him down to about 50% after about a minute and a half. When he starts floating again, use Slow Time and Multi Arrow. Do not worry about the minions he summons. He kills them himself most of the time. Keep moving and shooting, and your done.
-For his second form (dragon), head to the Temple of Avo and donate at least 50,000 gold. You should get a weapon called Sentinus. It has 255 attack and silver and piercing augmentations. This weapon can kill minions in three hits or less. As soon as Jack lands, use Slow Time and hit him on the head as much as possible. Use Berserk if possible -- the fight will be much easier. The only time it is safe to attack him is when he is in his original position across from the door. At any other time, kill the minions and avoid his attacks as best you can. Slow Time and keep attacking him to defeat him in under two minutes.
-When facing Jack the second time, use the following trick to dodge a lot of his attacks. Use Assassin Rush level 3 or above just before the attacks get to you. You should be almost completely unharmed. Note: The only attack where this will not work is when he makes a wall of flame opposite of his favorite side.
-To defeat Jack of Blades in dragon form faster, have a powerful longbow (Master's or Skorm's) and max out Multi-Arrow. Stock up on Will Potions. Take out your bow (if it is Skorm's this should definitely not be a problem) and turn on Multi-Arrow. When you can, lock onto Jack of Blades' head and draw back your bow to the fullest. Release it and it should do quite a bit of damage. Dodge his attacks as best as possible and ignore all the minions, summons, etc. that try to attack you. Use health potions and the Heal Life spell as necessary.