Trumpeter's Debilitate would probably be the best debuff overall, as you really only ever debuff bosses. And if you skill card the three Auto-ma-something-kaja's on him you can basically just open battles with him all the time. You need a Trumpeter with Mind Charge to max the Empress link anyway. --- Official Lopunny of the Black 2 Boar . . . wait, how did THAT happen?!?
You really shouldn't need Yoshitsune where you're at. It's not really worth grinding in Void. I'd just wait until you hit the next rescue mission, go on a rainy day and hit a few gold hands. You've basically already beaten the only "difficult" boss until you hit the true-end boss or the secret NG+ boss.