I use my Fleet of Doom backed up with a 'supply fleet' right behind it to replenish any loss.
Blitzkrieg used in WWII, the more planets I quickly grab, more money, snow ball effect. The other player has no chance to ever recovered.
My supply is not stretched out since I am slamming the planets with troops for more income as I go (Galactic Time is stopped and you get a bonus income for taking a planet pays for itself). Plus as the Empire, my Tie fighters and Bombers are free and respawn too.
Then you get this message "Player left game".