This is one hell of a tough enemy reminiscent of a dark souls boss.
I personally struggled to get the health more than half way. I only had 1 health potion which made things worse.
First off if you are struggling, check your resources. You can buy tainted blood for gold on the crafting screen. I was able to use gold to craft one more health potion, but importantly 4 mana potions which are significantly cheaper.
The key to this battle is switching from close combat to ranged. In the crafting screen make as many crossbow bolts as possible; I had a total of 23. Make a save after crafting so you don't have to repeat Every time you die.
As soon as the battle starts wait for a chance to hit him while he's busy butchering your allies. Switch to daggers and hit B whenever he does his ice attacks or if he swings directly at you, not only do you dodge you get a cheeky counter attack. The daggers movement bonus easily outweighs the reduced damage.
Sprinkle the first part of battle with ranged fire orbs and as many melee attacks you can. When health is halfway a cutscene then appears with help for the boss. Deal with the small enemies usually 1fire orb and counter attack will do.
After they are gone unload on the guy first and use all your mana up firing orbs then unload with the crossbow. Keep distance. He should only have a small amount of health . Use counters and allow your mana to recharge and your onscreen help should be able grind that health down.
The key really is ranged attacks and keeping your daggers drawn to dodge/counter.