I tried two set-up for killing Fengrus, a mage/warrior combo and a survivor/warrior combo.
In both cases, the Death Knight is the most important factor as he will go up against Fengrus toe to toe, and he has the higher health. Either boost his defense/evasion with Agility/Speed/Survival or health with Consitution. The one skill point which you get shall be invested in specialising in a weapon's damage.
The riftrunner is straight forward. I picked elemental missile attack over elemental instant attack because missiles does way more damage. For the mage, I pump every skill points into Intelligence (or you could shift more points to Intelligence at the beginning).
For a survivor's build, the skill I picked is of course Poison. It works with missile weapons too! (Hmm...would this be unbalanced?) You shall be able to find a bow at the beginning, with arrows. If you run out of arrows, switch to a club and start smashing. The key is to make sure Fengrus stay concentrated on the Death Knight.
