lol I saw this one on 4chan some time ago. There isn't really a trick to finish the game (I think it has 14 levels, or was it 15? whatever...). All you need is patience and keeping your head cool so you don't give in and shoot the damn dog. And some brains too, so you don't get fooled by some of the levels.
FUN TIP: Don't cheat. Always watch the whole levels until the end instead of leaving the computer and letting it go. You'll get a lot angrier and it'll feel great to shoot the dog 15 times in a row after you finish the game the first time.