I like the game but the fell sans boss was suck a brick wall, i did manage to beat it but with only 2 hp on susie, it was such a learning curve compared to the rest of the snowdin bosses
if it helps anyone, my strategy was to attack with kris and susie and distract with noelle, save the tp for dual heal instead of red buster incase you get damaged badly, it's slow but worked for me. also give everyone hard hats and permacicles