Comet Crash

Enemy units killed within a "box" are cumulative. You can box in 100 units, then box in 75 units, and then box in a final 25 units (200 units total) to unlock the Box Kill 200 trophy. You do not have to box in and kill 200 units all at once. The Box Kill 20 and Box Kill 50 trophies unlock in the same manner. Make sure you are in the proper area as each trophy is assigned to a specific one.

These trophies take a little planning, but are not difficult. The basic strategy is to create two paths to your base for enemy units to follow. At the same time, you need to ensure you have enough defensive units to protect your base. The following video will show you the basic concept;