i only let the pit lord hit the doom lord once then immediately use demonic gateway as i make the pitlord go back on kanrethad. used it every time i saw the doom lord[s] target me to drop threat. i think the aggro of the doom lords is tied to kanrethad himself, and the pit lord seems to lose threat pretty quickly if i didn't use soul shatter and demonic gateway as much as possible. my memory is kinda fuzzy but i sort of remember doom lords never attacking me during the time where kanrethad is about to cast chaos bolt but ends up not doing it since the pit lord has sufficient threat, but during the time when he actually does start casting chaos bolt, i use teleport then run up to my demonic gateway to wipe threat, causing the doom lords to go back to the pit lord after they finish casting their shadowbolts me [edit: as an example of threat mechanics of the fight. sometimes i'd gain too much aggro at various points but a simple gateway jaunt would fix it]. for the felhunters, i put the pit lord deep into the cubby as it seems like the pitlord has a hitbox that's just large enough so that the felhunter can see it and eat the enslave if it's not deep enough inside. never had trouble with felhunters after that [also there's only 2 felhunter phases].