If you're trying to fight off superior numbers of superior ships, you're obviously in a bad spot. Most likely, you've been drastically outpaced, and the AI has consequently gotten ahold of powerful combat techs. Very likely, they also have a more robust economy and manufacturing capacity... does your opponent have more planets?
Regardless, if you find yourself in this situation, you have a few options.
1) Forget building your low tech ships, they're just gettin whacked, right? Focus everything on getting more powerful ships and weapons, and start cranking them out once they're powerful enough to outclass the ships coming at you. Unless your tech disadvantage is absolutely crippling, this should eventually work.
Keep in mind that the main thing saving you here is that even if your fleet is all but wiped out, the AI isn't very aggressive about landing transports on your planets.
2) Use your existing ships VERY carefully. Don't send them into fights that they'll lose unless you really need to take out the target. Instead of trying to fight the warships that you can't outgun, go for transports, starbases, and traders... every soft target available. This works much better if you're faster.
If you're playing Dark Avatar as the Arceans (with their first strike ability) you can still fight toe to toe with the enemy ships as long as your fleets have a) enough firepower to take out many of the ships in the first volley and
you manuever them to have first strike in all battles
3) Ignore the fighting altogether and just use your transports to take his worlds. Defenders tend to be old, outclassed ships, so use your existing military to crush them and take the planets for yourself. If his empire is cut in half and he still thinks he's winning because of his bigger military, well... the joke is on him, isn't it?
