While the solution above me will probably work just fine, i have found an easier way to unlock the achievement.
With my solution you will only have to move one character instead of three at the same time.
Place two characters close to each other at the far right side of the bride, where the miner won't throw dynamit at them.
Then select the third character, pick up the bucket (lies on the bridge) and catch the dynamite. (Important note: At this point the bucket must not yet have water in it!) When you have catched the dynamite, run to your teammates and wait a second or two...
BOOM, Achievement unlocked. :-)