Terraformal Event is the fourth event in the Space Frontier booth in free play. To unlock it you need to complete the previous three events, or you'll get there eventually in Adventure Mode. Because of the luck factor mentioned below, I highly recommend you try for this in free play.
This is an annoying achievement, because, as mentioned before, it's partially luck-based. The luck component is that you may not get a TNT Powerup in any given play. That's annoying. You also need to prepare the area for terraforming, which means you need the powerup not to appear until you're ready. You can mitigate that with practice.
In the field, there are a number of targets that are attached to the tops of mountains. If you hit these once, they'll turn into two targets each, and if you hit both of those, a tree will grow. That means it's terraformed.
What you need to do is hit all of the initial targets once, so that all that's left are the sets of two targets. Don't worry if you hit one out of the two targets in any pair, since that's not "terraformed" yet. Then just sit until the powerup flies out. If it's not TNT, suck it up and hit start and restart the level. If it is, carefully hit it, which should destroy all the targets on screen. Because you set things up carefully, all the trees should now pop up and so should the achievement.