So let me see what's going on here...
This user wants a mod to ply as Metal Sonic because they cannot defeat Metal Sonic to unlock him normally. I'm afraid we cannot help you in that regard, you will just have to play better! But you can learn how to do that by watching others play. Check out peoples' playthroughs on Youtube for ways to beat the final stretch of the game. I'm sure you can do it if you just keep trying!
In the meantime, I'll just be locking this up, as making a whole mod just to play as a character already in the game is rather excessive of a resolution...