Rat on VC was pretty easy. JJ is just plain awesome for them, so I went that route, it did get a bit messy with some of the early minor punishments (Poison AND Mana Burn kind of sucks...

), but after the first level-up things were back on track. Thing is, high-health bosses are pretty meaningless for a Rat. So are resists. The only bosses that I find somewhat difficult are the ones that inflict Poison / Mana Burn (i.e. Jörmungandr, Tomithy, Tormented One), or do massive damage (Bleaty), but these usually have low health so can just be fireballed to death...
"Thinker", just without the "ache".