On my first layer 8 run I got the power that increases damage by 25%, but reduces max health by 12,50% (i think those are the numbers?). Every time you go up a floor you get one more stack of this. By floor 18 this power alone gave me close to 300% damage increase, but a lot less health - which I compensated by getting a ton of health increase powers. Had about 20k health at layer 18.
Combine that with double Convoke powers (100% dmg increase on Convoke), plus 3 50% multipliers, pluss all other powers made last boss die before Convoke ended with over 2m dps.
As a balance druid I love the moon fire > full moon power build. It's great.