Average level - pick bat or something flying. Just fly to the finish.
Dragon, boss level - pick necromancer. Summon skeleton, hide, summon, hide...
??????
PROFIT!!
949 seconds, guys. Was holding one of the early big ones at the screen for 2-3 min. Earn shield when it possible, don't hope on another turrets - protect only your building and don't shoot stuff flying away from your turret. Push asteroids at left or right not up. Also I didn't use spacebar shots, only left mouse button.
You'll get hard badge after some trying ;)
I am just uploading it now, takes some testing. Give it a try now.