Instead of the full RPG, many versions on Unblocked Games 66 feature the Bad Time Simulator , which lets you practice the notoriously difficult Sans or Undyne boss fights directly.
If you want this adapted into a longer chaptered story, a roleplay starter, or to include specific Undertale-like motifs (monsters, puzzles, pacifism vs. aggression), tell me which direction and I’ll expand it. undertale unblocked games 66