Save Room

Not my solution, found on Truesteamachievements. Credits go to chrisjwmartin

This can be achieved on Level 22:
1. Combine the red and yellow plants to make an R+Y potion.
2. Combine the R+Y potion with the green plant to make a sweet health potion (G+R+Y).
3. To get this achievement, eat a single rotten egg, then the G+R+Y potion, then the other rotten egg, then fill the grid with your guns. This will prevent you from completing the level on this try but the achievement will fire once you fill the grid.
4. Press P to restart the level, repeat steps 1&2, and then eat BOTH rotten eggs immediately and only then the G+R+Y potion, then fill the grid with your guns to progress to the next level.

The earliest you can do this, is level 11. It’s the first level with a first aid spray. Simply try to finish the level doing everything you normally would without using the first aid spray.