As Gnaw on Sorona. Using his "Ultimate" ability will grant you an ability called "Grow Weedling". You can get "bag of seeds" from the last choice of this abilities upgrades for an extra weedling to lay. But basically, just load up a training mission and DO NOT touch any of the settings. (Messing with the settings turns off achievements) You can lower the difficulty, but that's all. Choose Sorona as your map and Gnaw as you character. Once the match begins, run to the center of the lower lane. Place a single weedling, hop on the platform above and the button will activate the monster to eat it. Do this 10 times and the achievement is yours. Here is a video to help:
credit to HCmacko for the video
SIDE NOTE: The achievement didn't pop for me until I exited the game. It then was placed inside of my steam achievements when I logged back in. So, if it doesn't pop in game, lay 15 weedlings just to make sure your count isn't off, finish the game and full quit. When you log back in it should be there.