Too Hot to Handle

Aggro him to middle with the circle of fire/light. Let him stand there until 15 or more stacks. Kill him.

Easy achievement. Follow the fight course as intended, when he's around 250k hp stop dps and let the tank get him to 15 stacks and finish him. Use defensive cooldowns after 10-12 stacks.

Edit: forgot to mention, don't rush to get from 0 to 15 stacks, his aoe will kill your party. Patience is key! Good Luck.

This achievement is earned by letting the debuff stack on Karsh 15 times before killing him. Each stack will increase the damage he takes by 5% while also increasing the damage that you take from his AoE. The AoE only happens whenever he is 'dunked' in the fire pillar or is standing on a lava patch spawned by the adds. You do not need to leave Karsh in the fire pillar to quickly get him to 15 stacks. Instead, you should try to 'dunk' him in the fire pillar to stack 1-2 debuffs at a time. If your tank does not feel comfortable with dunking him in and out of the fire pillar, then you can have one of your group members stand on the opposite side of the pillar and taunt/distracting shot him before the debuff wears off.

The debuff only lasts for ~15 seconds, and once it wears off he will revert to quicksilver armor mode rendering him almost completely (99% less damage) invulnerable to your attacks. Every time the debuff falls off, the surrounding grate will erupt in flames dealing heavy damage to anyone who is standing on it and spawning 3 imp adds. The imps do not need to be killed, but if they are, they will drop a rather large lava pool on the ground. The lava pools act exactly like the fire pillar does (stacks debuffs on him) and if you don't move him out in time you can quickly overwhelm your healer.

Stay alert and slowly stack the debuff up and you should easily earn this achievement.