Destroyed a tower with a building projectile
You can play as either race for this trophy. Have player one and two each spawn outposts near each other. Then as player one, attempt to spawn an outpost so that it would spawn directly on top on player two's outpost. If done correctly, the outpost will fail to spawn but will damage player two's outpost. Repeat until player two's outpost is destroyed and the trophy will pop.
For visual aid refer to the picture below: