Ye Olde CN Games - Scooby-Doo: Scrappy Stinks (REDO)
I found out from Kevin Norwood that the game ends after hitting Scrappy a certain number of times and your accuracy will be rated. So I tried and it seems hitting him 29 times will end the game.
However, I managed to hit him with every shot and did not hit Shaggy or Scooby-Doo but the final accuracy rating was only 98%. My guess is you got to hit Scrappy before he disappears from the screen when he runs from one end to the other. This happened right after shot 21 (1:35) where he appears at the bottom left but retreats right after appearing.