Just beat Krosis with my levlel 26 Nord sword/shield warrior yesterday.
I had about 280 health and 200 stamina (IIRC, too lazy to launch the game to check, but basically I had way more health than stamina)
Kept pwning my ass a lot the first few tries. Eventually I brought Lydia with me (outfitted her with a full set of Superior Steel Plate armour which I had collected during my adventures, oh and she had a basic hunting bow and Exquisite Steel Sword)
I approached the coffin from behind/side of the word wall... as soon as he springs up, I took a few cheap shots (power attack first, then kept slashing) at him as his spawn animation cycle is being played. I had to do this because I am doing very small amount of damage to him, so every bit of advantage is important to me.
BTW, I am in full Exquisite Dwemer gear (armour and weapons), all unenchanted. No magical resistance except for the Nord's cold resist bonus. My Exquisite Dwemer Sword was doing about 35 damage.

So how did I prepare for this fight? I brought a LOT of Health and Stamina potions. Since Krosis has an annoying tactic of backpedalling and lobbing fireballs at me, the fight was a pretty intense case of tailgating him and slashing at the same time.
Two things that helped me take more life off his health bar are power attacks and my Power Shield Bash perk... pretty sure it manages to briefly stagger him sometimes (but not all the time).
so basically, chasing and slashing, then opening my Favourites and drinking potions to keep myself alive.
Eventually I had to hide behind a rock because he was a little too far for me to chase and I hadn't enough health - if I continued the chase, I'd surely be dead. If I drank my remaining potions, I wouldn't have any left to finish the battle.
So I hid behind the rock and pulled out the basic heal spell. I have no Restoration perks so the healing spell depletes my magicka so fast. Good thing I saved a few magicka potions. Also, my magicka is at 100 (never leveled it).
Role-play wise, I imagine that the Heal spell is a very basic spell that almost everyone knows (kinda like riding a bicycle or nailing something with a hammer) but in my warrior's case, he's not very good at magic (hence 100 magicka and no magic perks).
So while I was hiding behind this rock (which is higher than the elevation that Krosis was standing on BTW, so his fireballs couldn't hit me... again, cheap, I know), suddenly his attention turned over to Lydia and he started attacking Lydia. Finally the tank gets some use. ;P
While he was busy fighting Lydia, I took more cheap shots at him again. Finally his health is almost depleted.
But then, Lydia went into the "wounded state" (but she hasn't died yet) and then Krosis focued on me again. But this time, I managed to get him stuck against the word wall, so he can't backpedal anywhere.
His fireballs did hit me HARD even at this close range, and I eventually finished ALL of my potions but... when that final power attack landed on Krosis' head... nothing felt more satisfying.

Upon investigating Krosis' remains, Fjorin Heavy-Legs discovers an ominous-looking mask. Lydia, having just recovered from her wounds (she had some potions on her), warns Fjorin not to mess around with dead bodies, more so the remains of an evil Dragon Priest, but Fjorin picks it up anyway, telling her "worse come to worse, I'll just sell this off to some crazy spell caster..."