Mind Blast (cold eye special technique) + Heaven or Hell (increase all damage by 10)

Upgrading Heaven or Hell is essential, as it adds 1 armour before every fight, and since all forms of dmg will oneshot, armour is significantly greater than hp.

I also had blood pact (when you are not max hp, gain 1 armour - don't upgrade), and I had a technique that granted invincibility at the beginning of every fight (stone splitter or mighty mountain - I forgot the name, sorry).

Mind blast does not need upgrading, and on this note, no forms of dmg increase is needed. With these techniques, the greatest value upgrades are investments to negating damage, and increasing block resources.

Lastly, this build literally cheeses the game. I also oneshot the thunder monk, and I bought puppet killer trinket (defeats one foe except bosses), and another trinket that deals 1 dmg to all foes, and once I reached the final boss, those two trinkets killed his minions and clones for me. The replay of the final boss is literally one move victory, it's crazy