Tone down the GFZ3 boss's AI. I don't want the first boss of a Sonic game to be one that requires you to outsmart it to not take forever.
Instead of running away from you in all three dimensions, he could be at a constant height throughout the match, where you can hit him from the elevated border, but not from the bottom floor, and when you hit him, he runs away horizontally to the other side of the arena.
This keeps the "difficulty" that his running away poses, and allows new players to learn to approach things from a different angle once they find that they can't hit him from the bottom section of the arena. Knuckles players might also need to learn how to climb to hit stuff, depending on how high the boss is placed. On the other hand, you don't need to do a perfect run and immobilize him in a corner for the entire match for it to take less than a minute.