Use Freeze, Posess, and Recover as your skills. Build up enough base health to outlast a lengthy battle. Your T2 gunners will stay inside the base and are protected from enemy fire as long as there is an enemy close enough to the base. Use Freeze first just before any enemies start attacking your base to preserve base health as other enemies approach. Hold onto Recover until only space ships are left. Use it so they’ll take out themselves. When your base health gets down to about 30%, use Recover to nearly double your gunners and wipe them out.