How could Fierkraag give you trouble? I have never had any problems with Fierkraag, no matter what class I have been.
I measured difficulty in "number of reloads" time. When I first bought the game I played as an Inquisitor. Once I got out of Irenicus dungeon, I did Waukeen's Promenade and then headed for the slums, where I got contacted by Gaelan who directed me to a fellow in the Friendly Arm, which requested me to clear his lands of Ogres...
To be short, when my party reached Fierkaag it was around level 9-10, and it took me at least 10 reloads to whack him.
He is easy when you are 13-15, and actually know how to use the spells available