Messmer the Impaler- No Hit, Rune Level 1, No Scadutree Blessings
I did this before Romina, and this might be the hardest thing I've ever done in a video game. The only thing that comes close is my first time fighting Devourer of Gods in the Terraria Calamity mod. This was 6 cumulative hours of the same boss, dying to the same 2 attacks over and over again. I don't think his difficulty is very well designed in a no-hit/RL1 context, but of course, that is not what the developers had it mind, and it shouldn't be. Every boss should be possible to no-hit without having to factor RnG or additional help like specific talismans or ridiculously specific dodges that give up punish windows (cough cough radahn cough cough metyr) but I don't think they should specifically be designed for this, since its something probably less than 5% of their players are actually going to care about. I still think he's a great boss. Not s-tier though.
Again, it says enemy felled because I'm playing with Seamless Co-op for the extra save slots, and that's a bug in that mod. I promise it doesn't change anything else.