Now that you have your army, focus all your forces on one planet at a time, on whichever planets have the most spaceyards so that they can't retaliate as easily. Ground units are much more effective, since they can destroy buildings quickly, but gattling cannons can make landing a problem. That is why you need a large force. Continue this, quickly colonizing each planet as you go, so that the enemy can't come back to them. Do this quickly, and victory is assured.