It took me about ten deaths to find a workable strategy but I did it )

My strategy was to constantly spray fire from the Dragon Priest Staff (making sure I topped up its enchanting with soul gems) while constantly running or backing away from him, ie just blanket the whole area (or nearly) in flames. Marcurio's shock spells also helped. All the time also making sure I drank an ultimate health potion when my health got low.

I gather he's fearsome. Well done

What difficulty were you playing on?
Are you on pc, with mods or console (therefore vanilla)
Just curious!

Ah...I hadn't tinkered with the difficulty settings in the whole time I've had the game. I didn't even know until I checked just now, yea it's novice unfortunately which is obviously the factory default, unless I changed it to novice and forgot to change it back.

I'm playing on Xbox 360, no mods (only time I did that was on my Wii during the Wii's popularity years.)

Congrats! Now use those three summons wisely!

I thought the default was Adept? Can't remember, I switch it to Master (am kind of saving Legendary for the future) but I never had a chance against old Karstaag. I don't use potions in melee so that makes it pretty tough to beat him.

Now that this is in Cheats, Hints, and Spoilers...

1. He has a weakness to Fire.

2. He has no Ranged Attack.

Getting Him to spawn and surviving the spawning is harder than killing Him. He can not stand on a narrow ledge and pelt us with a Staff of Fireball while staying out of range of Our Weapon and Cloak Spells.

My first character to defeat Karstaag had a Nordic Greatsword and a Flame Cloak, and Perks in Destruction that made Her Weapon and Cloak strong enough to defeat Karstaag before running out of Health at Level 37. I wondered what all of the hype was about?

I learned on My second character to face Him.

Did Mercurio survive?

Adept is default fyi.

Grats. Karstaag is definitely a bit of a shock the first time you fight him. One of the better one on one fights in the game.

In all the time I played Skyrim I never used any of the summons I had, bit of a waste looking back!

I think by the looks of it adept is the default, my bad on that. In any case defeating him at level 43 when the guide recommends 50 or higher is something I'm quite pleased with.

And Marcurio did survive! I actually thought he did get killed coz I didn't see him through the fire during the whole fight. Good thing too as he's carrying a lot of powerful ammo that I'd been saving for any specific fights later on, and my inventory is chock full.