Aperture Science

The one real skill test in the game. Gold on all 18 Challenges - the tight thresholds on Least Portals, Least Steps and Least Time across chambers 13-18. Bronze and silver (Basic Science, Rocket Science) fall out of tidy play, but gold needs genuinely optimised routes, especially Least Steps and Least Time on the later maps. Don't brute-force the nasty ones - look up a per-challenge gold route and copy it. This is comfortably the hardest achievement here; everything else is a formality.