1. Ask some friends for help. Lots of people are really friendly, and are willing to help.
2. If you have crowns, get some henchman. Whenever I have a hard boss and none of my friends are on, I use the opposite school of the boss to defeat him. (The Henchman usually attack the first enemy on the opposite side unless they've been taunted, so when starting a battle, try going straight for the boss.)
3. Wait until you're a higher level and have better gear to take him down. Ignore the quest for a while and do other quests in different worlds to level up faster, and maybe earn new spells and clothes.
Also, if you still need help, I can help you, because all my current quests are dungeons, basically (like Briskbreeze tower and Nastrond.)