I always play game 6. I go to the topmost platform and stand at the wall on the left side. There I am pretty safe. As soon as a key appears on the left side, I go jump into the door.
Nice strategy

. I tried it while and felt is really possible to do great scores this way. Unfortunatelly, Iīm boulder breaker compulsive. I canīt avoid to chase a rock!
I use 6 too. Only difference though, once I have a key, I move to middle levels to smash holders until they start moving really fast. Then I head for a door.