Gorefiend has to eat a certain number of eligible players at a time or else the boss would reset. To make the boss even remotely possible to solo, the mechanic is just outright disabled if he’d eat enough players that he’d reset.
I did this achievement with a another guy (a hunter). We followed the strat from this video.
https://www.youtube.com/watch?v=NrJrYR9sduI&ab_channel=TrophyGamer
Basically my Lock spells (soul stone, burning rush) was what allowed us to succeed. One of the tricky parts was to kill myself. Even after reducing my health at 1% the boss was not strong enough to kill me so I had to remove my gear completely.
We tried easy heroic and mythic.
Still up for trying this if you want a 3rd
It definitely didn’t work in mythic for me. The npc we were supposed to kill never became targetable. I’m not sure if we did it in normal. My group was 50’s. He would probably die too fast in normal.