hey man, I just happened to see a thread of your where you said it sux you have to start a level over if you mess up an achievement. I just wanted to help you. If you are on, lets say the slingshot since thats what you used for an example, and you mess up. Just hit start then go down to save and exit. It will save your game and exit to the load screen then just resume your game again and it starts you at your most recent checkpoint. As a matter of fact I did it on the slingshot part, as well as several others. Thats all there is to it. Good Luck.