Excelsior

apparently there's a few bugged tiles around the area of the nujelm bio champion spawn where monsters can't be targetted or attacked

Those spots are definitely bugged. I think using Volcanic Eruption hits them, but not certain. They eventually wander into a kill zone.

we waited about 5 minutes he didnt move from his spot

There was a similar thread to this, talking about dragons in TK. Basically there are just some spots where it's going to happen. Volcanic Eruption or Earthquake takes care of them.