It has been a long time since I ran Pathfinder 1E, but I have this strange desire to run a dragon slaying one shot with a high degree of Player vs GM attitude. What do you think would be the optimal level to run such a game at, assuming that the PCs were to be built Core + APG?
That really depends on how much ammunition you want the players, and your dragon BBEG, to have. If you set it at, say, 9th level or above, then the PCs will have access to some degree of the old scry-buff-teleport combo, not to mention calling in at least modest reinforcements via lesser planar binding and lesser planar ally (or their higher-level variants if you set things at a higher level).
Also, it's from Ultimate Magic, but make sure to ban the synthesist archetype for the summoner!