I just defeated 99 enemies during the 10 rounds of the Ryoshima Coast Devil Gate and now the final Black Imp will not take any damage or attack me.

I have used several different types of attacks, including different equipment, Celestial Brush attacks, and damage scrolls. Nothing inflicts any damage on the enemy.

This is a pretty serious and annoying glitch as I just spent the last 15-20 minutes fighting enemies with the purpose of clearing this Devil Gate Trial. What's even more frustrating is that I didn't save immediately before the Trial. (I couldn't imagine encountering a glitch like this.)

Any suggestions for working around this glitch would be greatly appreciated. I did a Google search and didn't find any relevant results, so this may be the first report of the glitch on the Internet.

EDIT: If there's no way to work around the glitch, is there at least a way to exit the fight so I can save? I would much rather the last 20 minutes of gameplay not be an entire waste.

This is the first I've seen and heard such glitch.

Just a random suggestion but will it work after finishing another Devil Gate then returning back to this one?

It's good to hear that the glitch is over. It might have been one of those random errors that occurs once in a while. Despite that, enjoy the rest of the game.