I personally had a lot of fun with a Streetfighter/Forbidden Fist multiclass (if you have the community patch, it fixes the forbidden fist ability to actually be a weapon attack that benefits from sneak attack). The idea is that Forbidden Fists have the unique ability to deal self damage extremely fast; a Streetfighter multiclassed with anything else can sometimes run into the problem of not losing health fast enough before his allies go down and therefore needs to cheese the subclass mechanic with blunderbusses, sparkcrackers and cinder bombs.
Anyhow, you can go the safe route by being a Nature Godlike for consistent +1 PL, or you can take the fun route and be a Death Godlike which encourages not just being Bloodied, but actually Near Death, at which point you more or less ascend to godhood and have one of the highest single target damage potentials in the game (Mythic tier fists, Tenacious inspiration, chain attacks when you crit, -50% recovery time, ridiculous sneak attack damage, deathblows). You'll probably want to bring a priest with BDD of course. That said, you're fairly tanky too, so even without a Priest you won't die immediately even at low health as long as you don't have a ridiculous number of FF stacks.
Stat distribution: neutral might and dexterity. Dump Intellect, you want FF debuff to be as short as possible so you can quickly acquire wounds by repeatedly gaining only a single stack and letting it expire. Max out Resolve, then Perception (you want crits for chain attacks from Heartbeat Drumming and Swift Flurry), then put the rest into constitution.
On the rogue side of the tree Ring the Bell and Toxic Strike are useful in conjunction with the Enfeebled you apply through FF. Escape is obviously good for repositioning and getting out of dangerous situations. On the monk side of the tree you take Swift Flurry, Enduring Dance, Thunderous Blows and Turning Wheel. That's it, since FF has problems with generating sufficient wounds, and I like to save up Mortification to reuse SF/Enduring Dance when they expire. All your other skill points go into passive abilities, since both trees have lots of good ones.
As for how far you should push your Forbidden Fist ability, you'll have to figure it out yourself. 3 stacks hurts a lot but only gets you to around Bloodied. 4 stacks will pretty much drain all your health. A fun fact is that if you have your negative effects suppressed by a Priest or Paladin, you can spam FF like you have no stacks as well. It will cost 0 wounds, but the debuff stacks will still accrue and eventually activate when the suppression wears off. With Xoti in tow, I started a lot of fights with Devotions for the Faithful - Dire Blessing - Suppress Affliction, and then when it was about to wear off, use BDD because the debuff would otherwise OHKO you with a single tick.
For gear, I steal Tuotilo's Palm when I have ~10 ish mechanics on one of my party members. The Skaen Robe you find in the Hanging Sepulchres is really nice, as Might Affliction immunity prevents you from losing Tenacious, and the low health effect constantly triggers since you're often Near Death.
Out of the 6 or 7 PotD upscaled runs I've done, this build had one of the highest damage outputs. Great fun.
Josh Sawyer tweeting my screenshot of this build I posted on Reddit:
His courage thick as steel: