This is the most time consuming achievement in the game. It requires that you reach Level 25 with one of the characters. You should pick one of two starting characters at the beginning of Career Mode and ONLY use them so you are constantly working towards this achievement. The only way to earn XP to level up is by playing and completing Career Mode events. You can replay events you have already gotten a gold medal in to earn slightly more XP. The hidden gold medals are not required for this achievement. This will likely require a little bit of grinding, even after getting gold on all events. The scores you earn on each hole do no appear to influence XP in any way.