In fights like sapph where you know it can die easily you should have a macro that casts PW:S on it... sure that eats a bit of mana but it will keep it alive long enough to give you higher net mana.
I also find on sapph that I have VE up and that will heal the aura damage nps, it's more the tail swipe or cleave or blizzard that's a problem if I don't cast the fiend in the exact right spot... i generally try to do it on a 45 degree angle to sapph (from the back of course) and this tends to help him survive.
In all honnesty I don't need fiend mana that bad but the point is taken that it is perhaps a little squishy and could probably have it's health increased and some aoe resistances.