Use your space fighter (S.F.77) to destroy enemy spaceships and collect energy to buy upgrades. You could buy speed, homing missile, shields, etc. Doing so will make it easier for your S.F.77 to progress though enemy waves. Beware however of bosses and enemy flagships. These special space ships hove strong shields and could be destroyed by only attacking specific weak points in their design. The more waves you manage to destroy the more achievements are unlocked, and the more variety of enemies and attack formation starting to appear. So try to reach the enemy base and destroy it.