This guide includes a few items that really helped me fight Margit.
Rotten Stray Ashes
The Rotten Stray Ashes are really good because it inflicts scarlet rot on enemies, in my experience Margit will get infected somewhere between 50% and 75% of his health and it will last long enough that all you have to do is avoid getting hit for the rest of the fight, which is still somewhat difficult but not too bad.
Margit’s Shackles are a useful item that allows you to temporarily bind Margit to the ground a few times per fight. They allow for a couple of free attacks for you and any allies. You can find the shackles by going to the spot on the map. There you will have to fight a bloody finger, you get a cool knife from him too. Then you will enter the cave and fight some bandits, there is a trap of sorts there but I wont tell you what because spoilers.
Then you will enter a chamber where you will find a bald guy, you fight him until he is approx. half life, then you spare his life and he becomes a NPC that sells stuff, from him you can buy the shackles for a bunch of runes. This NPC also is somewhat important in the later game, so killing him is not advisable but won’t break the game too much I think if you do kill him.
I fought Margit along side some wizard NPC I summoned, he is cool and helpful.
I also used bloodhound’s fang, which is a curved greatsword you get from a boss in an evergaol. The skill is pretty cool and I generally recommend this weapon for anyone using a melee class.
Other tips might be go level up a lot if you are more of a casual player who is less adept at the game and these items don’t work for you.