Glad you enjoy the game. And I hope you don't give up because I think it will be rewarding to beat him..
Yeah experiment with plague. It is a good skill to use both offensively and defensively. Most of the effects are percent which means it just gets better in the higher difficulties.
Plague - necessary for % life reduction (not sure if boss affected they have so much life and its not like 50% or something anyhow plus I think they have a resist to it. Also necessary for duration to give you time to apply another skill and extends the duration of fatigue and susceptibility. Use this against mobs when you are at a mexican stand off with them...the plague will help you break it and all the monsters will stay plagued if you have enough points in duration because when one loses plague another will pass it back to him.
Fatigue - Small chance to do Huge % life reduction but more importantly reduces total speed of enemies by a % and the damage they do by a flat and I think a percent. The defensive part of plague. Helps keep wolves alive.
Susceptibility - Increasing damage to physical damage and elemental.....Max this since you are doing fragments and fire damage
The plague tree is better in legendary where the oponents get so strong that only extreme builds (unbalanced) can kill everything with ease. But a 12,12,12 plague can even rock mobs in act 5 Legendary. I haven't noticed it extremely good at boss but I have most experience with an illusionist and she kinda struggles at bosses.