Barebones Challenge!
MMX games provide all sorts of items and powerups to find and collect, but once you've done all that and beaten the game... they all seem to get a tad easy. The solution is to shun such things and take on the world using none of that optional stuff.
Beat Megaman X 1, 2, 3, 4, 5 using nothing but the normal X-Buster for the entire game, getting no capsule upgrades, collecting no heart tanks, and carrying no reserve tanks. Just default barebones equipment.
Additional terms:
Megaman X1 - If completing a stage affects another, complete the affected stage first. Enter Chill Penguin's stage only when all other bosses are defeated, but dash is optionally allowed when acquired as is X-Buster upgrade when forced on you later.
Megaman X2 - Do not defeat all 3 X-Hunters in mid-stage fights.
Megaman X3 - No Zero use. No armors or chips either!
Megaman X4 - Play as X. The game forces you to use your flame weapon against Reaper Sigma, so it's an exception.
Megaman X5 - Play first stage as Zero to get Unarmored X. Fight the right four before the left four. Always select "Energy" over "Life". No parts allowed.