Challenge% is an interesting category that blends low% with 100% in a single run. The first half of the run is just like a regular low% run where you complete the game up to the final boss with the bare minimum 9% items: Morph, Bombs, Ice Beam, Power Grip, Varia, Gravity, Space Jump, Plasma Beam, and a single Missile Pack. The first obtainable power bomb pack is at the very end of the game and you grab that as your 10th item, and from that point you turn around an finish grabbing 100% of the items in one big cleanup route before returning to finish off Mecha Ridley and end the game.
Quick Route Placeholder Description
"After PB's --> Skip the Workbot Super (save this for before Mecha), old cleanup route, head into norfair, ice beam area (make sure to get the ice heated room missile now), ridley shortcut, go to screw attack, enter kraid, full clear kraid, starting with speedbooster, and finishing with the missile below the rio room, go back into norfair, quick trip over to Norfair Space Boost, head to wave, enter ridley, spark up in norfair, go down to john missile, enter brinstar, full clear it, remember to get the ball canon missile, clean up tourian, then it's just old cleanup route plus workbot item"
Also throwing this in here as a placeholder since I made the vid already: