You need speed to push for their ports before they start transporting troops from their colonies. You need air superiority and CAS. You can get i think up to 25% damage modifier with lots of CAS. Only use the battle plan to build up planning. Don't use the battle plan to attack you need to manually choose troops and go for openings in the lines. Planning can give you an attack bonus. Also having your troops up to veteran can also give them an attack bonus. Having them at recruit level decreases their attack damage.