So far, I've been unable to complete this one. I'm pretty sure I know what to do, but the time limit is very short, and the ghosts won't co-operate. One time I was waiting for a whole minute and a half for a ghost to go and do his job. Another time one went south and somehow blew up a vital box.
Love the bridge building challenge in the middle there.
Like Loirae, in my plan I'm not using the boulder either.
Will go back to it later - I've tried a gazillion times so far!
EDIT: At last I did it! But only by turning the timer off ... and it took 9 minutes 5 seconds (I'm using my stopwatch) for the first ghost to do its job, and the second ghost to leave the exit area

Also arrived at the exit 1 coin short and missed my chance to get out!
A total stinker Keeks - but very compelling. Thanks (I think

).