From the beginning, levels can be tackled in any order, but the more levels you complete, the higher the difficulty (marked by number of stars in the level description) in all levels increases, keeping you constantly on your toes.
Etna gives you 10 hours to complete your mission, and as you beat each level, the count decreases. Eventually, day turns into night, and levels may be significantly different. In some cases, going to a level at night may even lead to a different boss than is present during the day. Repeated trips to the same level can also yield up yet more boss fights and different rare items to collect. The challenge of getting a better time and a higher ranking on subsequent playthroughs are further incentives to revisit a level, giving this game high replay value.
There's a lot to like about Prinny 2. I'm having trouble putting it down, just as much because of its inherent fun as the obsessive perfectionism it inspires. It's great strength is refusing to be easily mastered, and thus turning the game into a pointless slog through easy levels (or the RPG equivalent, endless gimme battles where all you do is click the attack button). But the flip side of this challenging gameplay is that casual gamers may quickly hit a wall and quit.
I don't want to call this a weakness. The developers made a conscious choice to make this kind of game, because there are plenty of "inclusive" platformers that anybody can pick up and muddle through. But it is important to emphasize that certain gamers will like it, and others will not. Even in diaper mode, the game can be unforgiving. If you're not sure which camp you fall into, I suggest downloading the demo from the PSN store before deciding whether to buy it or not.
Prinny 2: Dawn of Operation Panties, Dood! is rated T (Teen) by ESRB for Language, Mild Fantasy Violence and Suggestive Themes.