It's not impossible; actually it's not hard at all but it's about the tactics you use. Dark Matter mode is trickier as you will sometimes have to use different tactics, but you will probably have had to to complete the main game anyway.
Hint: Play it like normal mode at first. Then it becomes a brute force level.
Also, terraforming is an extremely powerful weapon, it just takes a little time. You should be making extensive use of it on many levels. There is even a clever trick you can do with terraforming that a few players discovered but I will not reveal that one.