Torchlight

To gain this achievement you need to kill Krag. This achievement is story based and hard to miss. (Close to the end of the game)

**Possible Spoilers**

After you kill the Ember Colossus you will meet Syl at a waypoint portal in front of another staircase that leads down. When you first enter the staircase there will be a cutscene from Alric's journal again.

After the cutscene you will be in the Molten Prison. This dungeon is rather packed with Goblins, and Trolls, and they are not an easy push over. There is tons of loot and enemies to fight along the way. (There are also suicide goblins that will run up to you and explode. Watch out for these guys, they have barrels on their back)

After progressing a few floors into the Molten Prison you will come upon a staircase at the end of Molten Prison - Floor 23. This staircase will take you to The Prison Stronghold. (Boss room)

The Prison Stronghold is also filled with Goblins and Trolls. You will do a fair bit of killing until you get to a rather large troll (Mini boss). He is not much of a challenge at all. This troll is also in front of the door leading to where the boss fight is.

After you kill the troll, pull the lever to open the door and proceed through. As soon as you cross the doorway there will be a cutscene of Alric talking to you. After he is done talking you will fight Krag in the open room.

Krag is very large and can do massive amounts of damage. He has a very large knock back attack, and he is surrounded by goblins which can get annoying at times. The real challenge is killing the Troll Masters. The Troll masters will run around shielding, and healing Krag. So it is recommended to kill all the Troll Masters first before engaging Krag.

After the Troll Masters are dead focus on Krag. He can dish out a bit of damage, but it can be avoided if you move before he slams his club, or by using ranged weapons.

(He is very easy to take when it is just Krag. Eliminating the smaller enemies first will help a lot in this fight)

As soon as Krag is defeated the achievement will pop, and you may proceed to the exit. At the exit a cutscene will happen. Syl will come talk to you again about what happens next. After she is done talking you can activate the Waypoint Portal and proceed with the game.

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.

Both solutions provided are great but I’d like to add something. I went after this one today and defeated the healing trolls first to ensure that Krag did not have his shield or was healed. After defeating the trolls his shield did not go away and I could not damage him. If this happens to you exit to the main menu and reload. When I did this I reloaded in the middle of the fight and his shield was gone. He’s easy to beat after the shield is gone.