Just started playing warlock (demo atm) a couple weeks ago.
Are there any dungeons where we should enslave a specific demon for a DPS increase?
Depends if you have supremacy or service.
There are only two demons i can recommend enslaving, and those are
Eredar Chaosbringer - Arcway. It gives a 30% dmg and a 30% haste buff for 20 seconds
Taintheart Deadeye - Darkheart Thicket. Makes a DOT that ticks for a total of 245000 dmg over 14 seconds