Again, Insanity gives a good guide. With plenty of detail.
This is simply what I did to defeat Krag.
As stated, Krag is very massive. In fact he is the FIRST boss that I had to actually focus on my skill set to beat. As such don't get upset at his insane amount of life and huge attacks.
Krag is a hack and slasher. He wields a club and is never shy about using it. As soon as the fighting starts, several Troll Masters spawn; these guys are a real pain. Krags life is huge and they do there best to keep it that way. You do not have to kill all the Troll Masters first, but I strongly recommend it. I killed all but one. I will warn you, when you go to kill the Troll Masters, Krag is on your heels swinging his club with massive force.
How to kill Krag:
I was an alchemist at lvl 25 (playing any side missions or having the adventurer perk, you’ll be lvl 26 maybe 27). I summon BOTH the Beam Golem and the Alchemical Golem before any fight to unsure they have full health and I don’t have to summon them while fighting. Also, for extra hitting power, give your pet a fish; any will do. Lastly and most importantly you MUST use Ember Shield to absorb damage delt to you.
Krag melees and Ember Shield absorbs melee; the little trolls do melee damage as well. If you don't think you need the Shield then leave a finger on LB at all times cause you'll need potions.
Once the fight starts your Golems will get to work. The Alchemist Golem will rush to attack everyone in the middle and The Beam Golem will stay fairly close, shooting everything charging you (hopefully), and your pet is lost in the mix.
Now it’s your time to shine. Any spells that don't have 5 or more points in them, don't use them. Ice shock isn't a bad idea to slow down the horde, but if you only have a couple points in it then don't waste the mana. Also, don't waste mana on Nether Imp either. They're to small to handle a crushing blow from Krag.
Focus on these spells in this order:
Pyre – Burns the minions trying to hit me till I'm down to half mana; also wounds Krag.
Ember Bolt - By now there should be 4 bolts that come out, either hitting 4 different enemies or slamming all into one (hopefully Krag) and pushing them back; do this till 1/4 mana is left.
Ember Shield - Finally ,I hit Ember Shield again. Trust me you've been hit several times and even though your health is full, you don't want to take that chance. Once you see your shield is back to full strength hold the X button. With a new shield you can take a break and either use mana potion and/or steal mana without much worry.
**Once you’re back to 3/4 mana or more start the process again. I socket my weapons to steal mana when I hit a target. This frees me up to use a lot of spells without focusing too much if I have enough mana for the next one.
This boss battle was the longest I've had so far. Krag is very well protected and healed vigorously. Make sure when your shield is up to run around and kill the Troll Masters or you'll be wasting a lot of precious time.
Krag is tough, but not impossible. The most important thing to remember is to stay out in the open. His hits are hard to escape. If you get stuck in a corner you'll be there a long time; if the Troll masters are out you'll be stuck there in a healing duel. It's best to keep moving till the horde is thinned out and then take Krag on. With a decent shield on and Krag having no help from his army he's easy pickings.