Your 'Beast' works similarly to a Pokémon, where stages act as encounters. Dependent on which grade you achieve in a stage your beast gains either a smaller amount or larger amount of experience. If you manage to get an A+ rating your beast will collect the most experience, once you gain enough experience your beast will evolve. Once you hit Level 7 your beast will reach its maximum level and pop goes your achievement. To give you an idea of what kind of grades you will need overall, my beast evolved to its highest form once I had 10 A+, 5 A, 9 B and 3 C ratings. You can quickly navigate to tracks using the TV screen in the menu screen (the bedroom) and by selecting 'X' - Your inner beast, from here you will be able to see what grades you have achieved and quickly select tracks. You can also check out other players who have already gained the max level and play against their ghost if you find that sort of thing helpful. Hope this helps.