Obviously you have a lot of possibilities with a villain who is being deliberately portrayed as unhinged. I think the line you need to walk is between making it honest and ruining the fun. One thought that came to mind was the reset. If the bbeg is on the back foot and it looks like the PCs got it, have the bbeg throw a tantrum and wish to try again. Everyone's HP resets, and maybe he had a few new special abilities to counteract the offensive moves that they first tried. Not to mention, this also goes back to a time when the bbeg has a wish spell. This makes it like a boss fight with multiple health bars, but there are some players who don't like this type of encounter vehemently and it can feel cheap. I wouldn't do more than 3 "rounds." If you party is really tactical, you might also entice them with new weaknesses that are telegraphed or maybe an environmental assistance.

Another idea that came to mind would be wishing they already won. Then you flash forward to the PCs about to be executed in some theatrical way, turning the encounter into a 007 escape that leads directly into the real boss fight. Maybe there are some low level horde waves, some skill checks, some absurd monologuing.

Best of luck!