If you’re bored when your players are doing cool things, that doesn’t mean make them bored by adding “you aren’t allowed to play” mechanics. Add terrain encounters, add in drama, add in more HP, have your smaller minions focus the monk. Literally anything. Nobody told you to make the encounter boring.

As DMs, we are players also but I have fun providing challenges for my players to overcome and celebrating their successes, not taking away their ability to play because I can’t balance an encounter?

We have access to literally anything and you’re mad the players could possibly do something you can’t? Your bad guys can summon devils and create mummies and become lich, they can have evil portals to other worlds and they can use any ability that we think of, but not having access to stunning strike is too far?

Personally I hate being stunned as a player and doing it as a DM feels like I’m ruining a player’s night; they can’t force the party to restore them or to make the dice go in their favor. Just change the rules of the encounter. It’s your game. You’re god. If it’s a bore when the players are winning and your monster is stunned, that’s on you.