D3 Achievements Guides: “Challenges: Act I” or “Instant Karma”
Have the Skeleton King kill 15 of his own Forgotten skeletons before killing him.
The Skeleton King has a few abilities, but the one you need is his whirlwind melee attack. It creates a circular blue effect around the Skeleton King, so it’s kind of hard to miss. This ability can also hit his minions, and you need him to kill 15 minions in the same kill run to get this achievement.
The strategy can vary depending on which route you take.
If you decide to do it on normal as you are leveling (you should be around level 7-9), then it will be a hectic fight. Remember, you don’t need the 15 minions dead in the same whirlwind attack. He can kill a few at a time and you will still get the achievement as long as they add up to 15 over the course of a single kill. I would suggest kill a few skeletons yourself because they drop health globes which you will need to stay alive, and generally steer clear of The Skeleton King. There is no reason for you to soak his damage. When you are ready to have the adds killed, position yourself between the minions and The Skeleton King. He will either move or teleport over to you, and you can then move out of the way when he starts whirlwind.
The easier way is to get to a higher difficulty level. My Witch Doctor is on Act II Inferno. I can just stand at The Skeleton King, and without so much as moving, I can get the achievement. This is because the spawned enemies and The Skeleton King will stack up on you, they won’t be able to hurt you, and every time The Skeleton King swings, he will continue to kill his own minions. However, you should be careful about thorns though, as thorns will essentially kill everything that hits you.