In my party we have a ranger, a sorcerer and a monk. We recently had a fight with a Balor and I had to keep introducing things in order for the monk to feel like he was contributing in any way.
For those who don't know, the Balor has a fire aura that deals 3d6 fire damage to any creature that starts its turn within 5ft of it, and it deals 3d6 fire damage to any creature that hits it with a melee attack while within 5ft. For my other two players this was fine, just stay away from the Balor and pelt it with ranged attacks, but my monk was dealing less damage on average than the Balor was doing to him so he just decided it was safer to stay away.
Obviously this is a feature of creatures like the Balor but I am looking for ways to ensure my monk feels like his is still contributing to the fight when they are fighting something that, essentially, kills him if he punches it.
I'm aware I could just give him an item that makes him immune to rebound damage (a term I just decided on) but I am hoping to find something official that could either negate or offset the damage he takes.