I did it at level 17, I had full heavy, with + to health and stam. Picked the mammoth and just stuck on em close, and did circles around em while blocking, when he did his special I stopped and wailed for a few, then repeated the process.
1hand and shield. and as much of the passives I could get. For my 1hand weapon it was a purple axe with speed and electricity for the enchant.

Shield had whatever was not overcharged, cant rememeber if I had a special, I was rushing it so I prob didnt.

Health/stam foods, health+stam potions, you can make those at 15 I believe.
Blue or purple.

Extra critical, and passives on 1hand to help with the block reduction.
Mundus for a few of the pieces of armor, picked the one with the increase to criticals.

Took a couple tries but eventually figured out his movements. Dont bother rolling out, waste of stam.

Even with all that though, I think it took me 4 or 5 tries before I finally beat em.
He has a pattern though, once you get it down, hit em, he hits back, hit em, etc. Special, hit em bout 4 times or 2 heavies on em, save your fear and pounce when you really need it.

If your gonna lose just back out a bit and he resets.
Bring some repair kits and a some of those filled stones.

Maybe someone has a better approach to it, mine was a bit a hasty and I could of done better on my armors and enchants. Lessons learned