Well the first time I tried to beat Fenagus i died <img src="/ubbthreads/images/graemlins/disagree.gif" alt="" />.
On the second try, however I was using a mage/warrior combination.
I used the deathnight as the punch bag and threw every INSTANT fire spell I could at Feanugus. He did, of course use restoration but then I had 5 mana bottles so I just kept replenishing my mana while I used the 12 health bottles to keep the deathnight alive.
So basically use your hero/deathnight as a mage and your deathnight/hero as a warrior and you should walk it. Just make sure you pick up enouph health and mana potions <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />.
:SPOILER:
If only there was a two handed sword at the beggining of the demo <img src="/ubbthreads/images/graemlins/think.gif" alt="" />.