I currently have a lvl 17 SS/Fire brute, and I find I can solo incredibly well with my difficulty set to Vicious (third one for +1 enemies and bosses).
Most of my stuff isn't even really properly slotted (Healing Flames only has one recharge in it) and since I just turned 17 I barely have cash for new DOs..
It's not that hard, you just have to not be afraid to use Rest quite a bit. I am thinking of slotting it, too, strangely; with one DO recharge in it, it's available every third battle. I can usually last that long, but sometimes I'm up against red bosses and it's a bit more complicated (I miss quite a bit).
If you build up your Fury adequately, you can quite literally SMASH red bosses, provided you hit them of course. Using that technique, with your Fury at 3/4 full, I take off almost half the health of a +3 boss with KO Blow..
BTW, all my attacks are currently slotted with one aim and 2 end rdx. I still have endurance issues if I don't watch out my Fury build and use Haymaker + KO Blow too much, but it should mostly be resolved by the time I have Stamina (in 3 levels).
