Of the two methods (Time Bomb and the stay on round 1 with chalkes method) I found the time bomb method the easiest and surprisingly less time consuming (apologies for the the pun).
All I would do was drop down to the remmington chalk, apply the chalk to the ? next to it, drop down to box and have a pop. If I didn't get the time bomb first try, quit and re-start.
It is possible to have two tries at the box, but if you get the time bomb first you can recapture all the early point opportunites (such as the 200pts when you free Leroy).
It took 10-15 tries to get the time bomb on the first go, but I think it is easier than running a loop in the cave system to get you to the bar!
K.