well i managed to find a strategy that works. First, use one flower to make ONE enhanced defence tree, and then use all of your seeders to make new dyson trees. when the first wave of attackers comes in and tries to take the asteroid between your first two, strike with ALL of your seeders and laser mine (the laser mine should be sent exactly when the enemy force first strikes in oder to coordinate teh attack properly) immediately after they break into the core, if you've waited until the core strength is down to 100, you're too late. Once you've taken the middle asteroid, send your remaining flowers to make more laser mines. The lower left asteroid will eventully attack your original right side asteroid, this is where you should have sent all of your laser mines (by now you should have around 3 or 4). Send all of your seeders at this point to help defend. After you beat this wave send all available seeders to the lower left asteroid, take it, and then plant all dyson trees. From here on out, there are no more huge waves and you should be able to complete the level.
(Unfotunately I was only 2 asteroids away from victory with a 400 seeder army and 9 laser mines when my game crashed... Oh well, there's always tomorrow to try again...)