that without an archon unless even prepped to the tits if the fiend is anything but weak your chances of killing it are next to nill. Magic missle doesn't hurt it and those 1 or 2 weak attacks a round generally won't do it. It can be done with and archon but I have had times even with transplendant and starting with full mana that I ran out of mana casting magic missle without killing the fiend and like 20 ticks later after using heal mana and still throwing missles the archon left and the fiend won.

That was my experience anyway with having many edges aimed spcecifically to help me against bards and fiends.