You gain experience by completing levels, playing matches, and completing the game's seemingly endless supply of quests. The quests usually just consist of beating a certain CPU level in a certain mini game. Keep doing this until you reach rank 25. I got the achievement around the beginning of world 5, but I had been doing a lot of quests on the side. Also, it didn't unlock at rank 25 for me. It unlocked when I hit 24 and a half. Maybe a glitch?