This achievement requires that you kill 20 workers with a single use of the Earth ultimate power, Rock 'n Roll. This is the power the spawns a large boulder to roll down and kill workers. You will not be able to kill 20 workers until you upgrade this power up to three stars from the Shop. Before doing so, the boulder will only kill 10-15 enemies before the rest will jump over the boulder. This can be done on almost any CIRCULAR Campaign or Survival level. Just let the workers build the tower to near completion (3 to 5 tiers) and then be sure there are many workers on the way to the top. Launch the boulder near the top of the tower. The achievement will unlock immediately if you kill 20 workers.