HK-47 is not that difficult. Here are things to remember.
You forgot the HK stealth/move/snipe bugged mechanics, leading to an invisible pillar falling on him even if he isn't anywhere near the center of the room...
... leading to death.
It seems this can be caused by AoE damage when he's supposed to be safe in the pillar, but it's not the only issue.
First attempt on him was solo with a barely decently geared 47 tank juggernaut, took me 4 attempts with all the bugs falling on me one after the other and the companion's control being quite hard (when not path-bugged too), wasn't perfect on the 4th but finally took him down.