orchid blossom
Explorer
Our 15th level party recently took on a Pit Fiend. We had a huge advantage though, the Pit Fiend was frozen in time. So we were able to do things like Consecrate the battle area.
We found him frozen in a secret room in our manor house, in the middle of a battle with a family member who was blocking him with her staff of the magi. He was in the middle of breaking it. Now, the fact that we had all the time in the world to set up the battle area like we wanted made a huge difference, but our basic theory should still help you.
Make him waste actions. I know, this is more party planning than DM help, but still. We did stuff like smash tanglefoot bags on his feet. Sure, he's going to break them no problem, but he has to waste time doing it. We used a rope of climing to tie his sword and hinder his swing, we even tied bedsheets around his head and tied him up with rope. We knew none of this would stop him, but after all that, and a few other things, he spent his entire first round getting all that annoying crap off him, which made a big difference since he'd won initiative. (We had the means to move time backward six seconds, so we set off the battle and then init was rolled.)
We found him frozen in a secret room in our manor house, in the middle of a battle with a family member who was blocking him with her staff of the magi. He was in the middle of breaking it. Now, the fact that we had all the time in the world to set up the battle area like we wanted made a huge difference, but our basic theory should still help you.
Make him waste actions. I know, this is more party planning than DM help, but still. We did stuff like smash tanglefoot bags on his feet. Sure, he's going to break them no problem, but he has to waste time doing it. We used a rope of climing to tie his sword and hinder his swing, we even tied bedsheets around his head and tied him up with rope. We knew none of this would stop him, but after all that, and a few other things, he spent his entire first round getting all that annoying crap off him, which made a big difference since he'd won initiative. (We had the means to move time backward six seconds, so we set off the battle and then init was rolled.)