I continued playing a bit, it works on SteamDeck’s ArchLinux but you need to download Windows version manually from website then add it as external game to Steam library and run it with Proton (I used Experimental). Probably just works with WINE on other platforms.
I found a bug where you can jump a little before the master talks where a bonus will be, then you still fall during the animation but after that you automatically receive that bonus anyway (and probably only that, it was the bonus that gives you the two other bonuses at half purity so in this case I had to pick the other two anyway).
I was glad to discover alternative inputs: arrow keys + ZXC which I use more often than WASD + JKL (although in my own games I define both exactly the same way!)
Apart from the crash which only happened once (see my other comment), my biggest issue was that I couldn’t use the blood arts at all (I found two, one said to use A/D so left/right + K or L, another to hold K to attract blood and hit L, but neither worked - maybe I had to release K just before L? I also tried the alternative left/right+X/C, holding X and pressing C… nothing did work).
Finally, a training mode where you repeat the same boss again and again (with or without the bonuses) instead of restarting from scratch would be great to get a feeling of progress. Either it would really be just a training and you’d have to start from scratch to really advance and unlock next boss and its training, or it could be a full “easy” mode where you just have a checkpoint for every boss.
I like the boss rush challenge but it’s getting less and less interesting to fight the early bosses so it’s more just taking time than a real challenge and I’m not improving either because I can already beat the first two with no blood heal (so purity 100) so I have no benefit in beating them with slightly more HP at this point.
So it may be worth waiting for an update before I keep going? So I can get the crash fix, the blood art input fix and maybe the training/easy mode if you can do that. I’ll try the other Boss Rush entries in the meantime.