Working in teams of three to build earthbag structures is often the most efficient. You could use fewer or more workers per team, but three seems like the most efficient in most cases.
Worker #1: Filling buckets, moistening and mixing soil
Worker #2: Carrying buckets of soil from worker #1 to worker #3
Worker #3: Filling and placing earthbags
Rotate jobs to break the monotony. This also gives you a break because you’re using different muscles. Everyone can pitch in to tamp bags and roll out barbed wire. Two or more teams will greatly speed the work.