After defeating all four bosses and collecting their Trophies, you will unlock the final challenge: Nitros Oxide. He will challenge you to a race on his home track, Oxide Station. This is the ultimate test of your racing skills.
Nitros Oxide is the fastest and most aggressive opponent in Adventure Mode. He has perfect racing lines, consistently chains mini-turbos, and uses items effectively. You will need to bring your A-game to defeat him.