So much fun! I played an old version and there were lots of ways to just cheese the game; this time I had to think even about when I should attack.
This is my final backpack to defeat King:
For the first many round use the whetstone to buff the assassin's dagger (which has an attack of 53, 2 energies) and use the manas to get +4 blocking from the ring until we have enough blocking. See that my mana doesn't recharge, so this is tricky.
use the extra weapons to kill any minions.
once the assasin's dagger has been buffed to over 100 and we start running low on mana, start attacking with it, initially 1 per turn and then twice per turn.
Once we can kill him without dying to spikes, attack with what is necessary.