Never give up fighting. u might just win even if the enemy destroys your base.if in space use your ships to attack the enemy
Rebel Space use ur X-wings,and Y-wings (A-wings too) to attack the enemies bigger ships they are somewhat weak against the smal fighters since their big laser can't seem to hit your ships(even works on bosses)
Rebel Land hide your troops then come out and suddenly attack then run away,or even better build turrets and lead your enemy towards the turrets(not recommended for the long distant enemy or AT-ATs)

Empire space use your tartan patrol cruiser to attack small fighters anduse any ships that you get to attack the enemies bigger slower moving ships
Empire land hide your troops or build turrets then attack using method mentioned above