Hk-47 isn't really that tough himself. it's the fact that you are constantly dealing with adds and it's a FP thats heavy on heals. Your Healer and Tank really need to be on point when dealing with HK and his constant flow of adds.
I've noticed that when HK goes to the center of the platform rendering himself invincible people tend to just stand there and stare at him instead of getting to the 2 corners where the adds appear so you can get the drop on them. This drive me crazy.
It's like, what are you doing taking a freaking photograph of the guy, get to the F-ing corners and get ready for the adds!!!