This Achievement can only be obtained after beating Leshy.
This guide contains spoilers for the post-game of Inscryption.From the Fandom (dot-com) Wiki for Inscryption:
(styling added by me for clarity and emphasis)
This achievement is tedious to obtain. Use the Energy Conduit and another Conduit to generate unlimited Energy, and use an Emerald Mox to play the Stim Mage, who can now be infinitely buffed. Don't bother to spam click the button, as it only registers the next click after the animation finishes. The Bone Lord's Horn and Bone Heap can replace the Stim Mage, but this is even more tiresome, since it requires repeatedly clicking both of them. Receiving this achievement will naturally give you an absurd amount of Foils, making it easier to get the achievement Collective Effort in the same run.
Summary:
1x Energy Conduit
1x Conduit (Any to complete the circuit)
1x Emerald Mox (or equivalent)
1x Stim Mage
It's easiest to do this at the
as it lets you setup without taking damage. It takes about 5-10 minutes to manually click the Stim Mage's power-up button due to the delay caused by the animation. The damage must be equal to or greater than 666 to unlock this achievement.