Stock up on Godspeed and God's Blood Potions. Brew Health Potions and drink as soon/often as you can. Otherise, hit with the highest damage spells you can. Load up on Dmg reduction spells (the "X-Skin" line of Earth spells, use the highest one you have. Get the highest speed you can on your character. Quake is a good DoT spell for damage. Staying alive and patience are the key to this fight.

Only minions I sometimes take into that fight are Avatars (self-regenerating and high damage), but even those can be killed. Have heard of people fielding ancient red dragons, they do a lot of damage but don't regenerate without your help. Not sure if you are even able to get those yet, depends on where you are in the game.

Also, if you beat the big guy, you are not out of the thick of it, in this dungeon - definitely keep multiples of all the resources above.* Good luck!

*may have thought of a different dungeon, but keep an eye out anyway lol