Here's some strategies that worked for me...

- Build up fleets inside your ports before sending them out to do battle, aiming to take on single enemy ships where possible.
- Until you achieve parity always try to end the turn in port too.
- Retrain every time in port to keep your ships at full strength.
- Move in short steps so that you don't find yourself running into an enemy fleet that was previously out of sight.
- Don't just attack the enemy fleets, take down their bases too. Send assassins to sabotage ship building facilities or even invade the settlement. If you can't hold it, destroy everything & then leave it to revolt.