Flock!

For this achievement you must use your saucer to Rip Up 50 Trees (Aggregate Total).

To do this, position your saucer above a tree and use (A) button to change between your modes of push and pull. You obviously want to use pull. Then hold Right Trigger (RT) to pull up on the tree and it will strip down to a stump. Sometimes you have to work with the tree while pulling on it by holding (RT) and moving in a circular motion around the top of the tree. Now the 50 ripped up trees will be counted towards the achievement only upon completion of the level you are in. At least all the other achievements in this game are like that. As far as what levels to use, if you just started this game complete each level with all trees stripped and you should get it early on. There should be a decent amount of trees in every level. Very easy Achievement.