For my strat, the best defense is a good offense. You may or may not want turrets to set up a defense of your assets while piling on the freeze/miner spam.
To elaborate on my strategy, the Miner Ship spam will earn you more energy with their draining attacks AND clear enemies and asteroids. Freeze Drones will stop enemies from attacking. Enough of both and you can build and attack simultaenously.
I'll post my epic difficulty stomping strategy.
(1) Equip Freeze Drones and Miner Ships
(2) spam Fighter Starports pumping out said small ships
(3) deploy Physics Labs to accelerate production and Constructors to increase buildspace
(4) get Reactors online and build Medium and Heavy Ships which can use the swarm of Freeze Drones and Miner Ships as a meatshield cloud.
Certainly interesting as a game. Nice art direction and it feels more complete than a lot of "sales release" games these days. Seriously just a couple dozen more levels and maybe an editor and you could sell this game as a DS title or something!
I didn't notice it until recently, but your first few heroes get much more experience than the others. This makes placement and slow abilities even more useful.
 "N (Official Web Version)"){kind=icon}
 "Outernauts"){kind=icon}
 "Give Up"){kind=icon}
 "Obliterate Everything 2"){kind=icon}
 "Glean"){kind=icon}
 "Nightmare Runner"){kind=icon}
 "Help! It's the Unfinished Shadow Game"){kind=icon}
 "Crystal Story"){kind=icon}
 "Protector IV"){kind=icon}
 "The Enchanted Cave"){kind=icon}
 "Enough Plumbers"){kind=icon}
 "Bubble Tanks Tower Defense"){kind=icon}
 "Blosics"){kind=icon}
 "The Endless Zombie Rampage"){kind=icon}
 "Onslaught2"){kind=icon}
 "Elona Shooter"){kind=icon}