This was actually rather fun, despite some of the hiccups I had. I think the hardest part for me was trying to read the part at about 2:18. That was about it.
Osu! is a rhythm game based off of Osu! Tatakae! Ouendan! and the American release Elite Beat Agents. In these games you click circles to the rhythm of the song. Osu! also has different rankings you can shoot for, with SS being the best (Hit every note and have 100% accuracy). THere are also mods like Hard Rock, Hidden, Flashlight, and Double Time/Nightcore if you want to increase the difficulty even more.