I mean I've played a lot single player, but can't beat unfair AI though
End of Killer7007's quote
Spam LRM, set your planets' rally point to your capital ship, and expand and attack quickly. Eventually build a couple frigate factories on more front-line planets and build from there instead. It shouldn't take too long to get ~50 LRM; once you have that, you'll probably easily plow through whatever fleet your opponent's built.
Remember to upgrade your civilian infrastructure ASAP after you colonize; if you don't, you start losing a large amount of funds to underdevelopment. Asteroids require one infrastructure upgrade to stop losing credits, full-size planets require 2 civilian infrastructure upgrades.
Oh, and I recommend turning pirates off if you haven't already, they're too annoying, especially with AIs with income advantages, and add very little of value to the game.