Fleets cost a lot of money and they cost a lot to maintain. My main opponent has a lot of fleets, each one with a lot of ships. No doubt he is spending a lot and he does have the best economy so far. Usually I attack the fleets and then the planets but i was wondering if there was another way. What if I attack his planets with transports using the invasion that causes the most damage to infrastructure and population? I'll just kill the defenders and leave his fleets alone if I can and then destroy the infrastructure to hopefully ruin his economy. His fleet costs will then further ruin him. 

Has anyone tried this against a militarily and economically powerful opponent? I know at first his approval will probably go up since he'll have less population per planet. I also thought I wouldn't even try to take his planets but instead just wear them down to a low population and quality, hopefully making little money. 

Some of his fleets have 8 ships. That's got to be about 240 bc per turn maintenance. He's probably got at least 12 fleets like that so thats 12 X 240 =  2880bc per week just to maintain his fleets. Then he's got lots of defenders. I already wrecked his starbases. I'll hit his planets and hopefully nail some of his economy buildings until he just can't maintain anymore and he'll have to surrender. 

I haven't had a chance to build any spies yet but I'm hoping that my tactics will get me some time to do that too. 

And if anyone wants to know I went back to an earlier save in my battle as Drengin and I researched good and evil and chose good for the many benefits, one of which is that I wouldn't have 4 civs declare war on me at the same time. This worked out well. Much easier to fight against one opponent while having other civs to trade with and even form alliances. Also, the empathy structure I built got two large civs to surrender to me so that certainly turned things around. I feel like I have a pretty good chance now whereas as evil I had almost no chance with my skill level.