Carrier capital ship backed up by long range frigates is always a great stand-by. This is a lot tougher for Advent, since they need 3 labs and their illuminator isn't as good at focusing on capital ships.
Of course, the #1 rule is to scout and grab your early planets very quickly. Depending on how far away the enemy is, you may have some time to build up, but only if you're quick. If the enemy is close-by, direct all your funds towards military.
