This plant can be found in multiple locations and can be used to launch objects into the air.

The game defines a critter as a bug of small animal. They can also be found in various locations but on this beach you can find several crabs.

Once you have found some critters deploy the flowering cactus on the ground close by, To get the best chance of unlocking the achievement it is best to try and get it at a slight angle however it is not required.

Once you have deployed the flowering cactus you will need to grab a critter and place it on the flowering cactus. If the critter walks off and is not launched try holding it until the flowering cactus is about to launch then let go.

The Critter should now be violently launched threw tha air.
Luckily for your conscious it shouldn't die unless it lands in the water or you found a critter on one of them floating islands like they had in avatar, If one of these did happen then congratz you murdered and inocent animal for an achievement, After what you did to the meep I'm starting to think you have issues.