What music plays in Hunter x Hunter SLOW MO?
Okay after quite a bit of research, I think I may have found some stuff that may help you.

The episode that you are looking for is Monster x And x Monster (ep. 112). The slow motion part is largely the first half of the episode.

There are a number of tracks that play during the slow motion scenes. Here's a playlist of tracks from the Chimera Ant arc (30 tracks), of which I think your best bets are either Hegemony of the Food Chain, Kingdom of Predators, or Legend of the Martial Artist. Of the three, I think Legend of the Martial Artist is the song you are referring to, but you can go through the playlist to find the specific song for which you are looking. Keep in mind that multiple tracks were playing during the time you are referring, so it might be a mix of a few different songs in that playlist.