If you don't have a co-op partner, and don't want to play through the whole game on split screen, there is another way:
1. Start the game in solo.
2. Before the end of each level save.
3. Set up a split screen co-op game; you can load your single player saved game from here.
4. Finish the level on split screen.
5. Repeat steps 2 to 4 for each level.
This method also helps to get the following achievements:
P.S.: Unfortunately, you can't do that when replaying levels after completing the game, since there is no save feature on replayed levels.