This achievement requires that you spare all jar bearers when you launch the Tidal Wave power. Jar bearers are the workers that carry the purple smoking containers. The Tidal Wave is the Water ultimate power. This achievement is actually easier if you do not upgrade the Tidal Wave power. This can be earned in Campaign (Level VII onwards have jar bearers) or in Survival. The tricky part about earning this achievement is you must spare at least one jar bearer AND not kill ANY jar bearers with your Tidal Wave.
This is easy to obtain in Campaign Level IX. From the start, do not do anything. Let the tower build up. Eventually you will need to fight some ships. After the ship fighting sequence, let the workers go again. A single jar bearer will spawn and walk all the way to the top. Right before he reaches the end of the built tower, he will drop the purple container and continue walking up. Launch the tidal wave power JUST BEFORE he reaches the end. He should be about four levels up the tower. The Tidal Wave will come and only destroy the lower section of the tower, sparing the jar bearer. No other jar bearers should be on the lower levels, but if there is, be sure to use another power to destroy the jars before using the Tidal Wave. Otherwise, the achievement will not unlock as your Tidal Wave will have killed some jar bearers.