For the last two levels I strongly suggest you to capture the turrets NOT AT FIRST but when your enemies start fighting each other, in the last one go straight for the jumper and the for the "megaturret" and try to gather about 400 ships (basicly do not attack the dead ring but try to capture as many outside planet you can and jumpers) and through a jumper attack their core.
Awesome compliment for this game to be mentioned in the same sentence as the awesome Civilization series jaalacba, Thanks for making my day! =)