I played the game to the end on the easiest setting and never saw a roller. A roller is like a yellow machine that would roll pavement in real life. There are two roller cheeves one for destroying one and one for dying from one. I restarted the game on the hardest difficulty and picked stage 7. If you beat 7-1 without dying there is a roller or two on level 7-2. Destroy it then reload and let it kill you for the two roller cheeves if you need them and didnt play on harder difficulty in the first place.