I have no idea if it's possible to hack the game in such a way as to actually activate the fight with Sheltem, or if just his graphics, music, and name are all that exist, since my ROM-hacking skills are far too limited to find out.

This game apparently has "squares" with set encounters with a particular types of enemies. I wonder if someone with more NES hacking knowledge could modify a save state to change the ID of the enemy that appears when you step forward a square into the set encounter.