There's a much easier and non cheating way to get that one, in that skirmish level where the asteroids are super fertile (I think it's called Abundance), plant 5 seedling trees on an asteroid (Preferably a very weak asteroid) and let the enemy conquers it, then swamp it with mines. Anytime an enemy seedling will spawn, mines will destroy it, after 15 minutes and 500 enemy seedlings destroyed that way, you should get the trophy. It's like Attrition, except longer. I forgot however if you must swamp it with mines or just send a single one so enemy seedlings aren't killed to quickly and the games still believe this is one long battle rather than several short ones.