How to make a good distract mechanic?

I’m going to be DMing for the first time soon, and I have a late game boss fight I’m still ironing out. The boss‘ ”gimmick” if you will, is that they have a body that’s basically just a suit of armor being possessed by a poltergeist, so when people try and attack, he simply moves his body parts out of the way. Now, I want a cool mechanic where they can distract him for a long enough time to deal damage to him before having to distract him again. How could I do this?