How to beat the HARDEST mode in Demon Slayer 2

One of the most difficult challenges in Kimetsu no Yaiba: Hinokami Chronicles 2 by far, having to defeat 40 Slayers / Demons in endless / endurance / survival mode in a row, ALL to unlock a single "secret" map, the Hot Spring. Fortunately, I have some tips and tricks for you to unlock the Hot Springs map, and maybe even get the achievement you need. I ended up in the top 10 on the leaderboards, though I used a bit of cheese. While it's possible to do this with slayers like Tanjiro, Zenitsu, Inosuke or even Nezuko, I recommend you pick a demon like Zohakuten or Gyutaro as they'll be able to use Surge / Boost to regenerate their health every round.