This guide is for those who are having trouble dealing with any kind of military difficulty. Even experienced players may learn a thing or two. So pay attention.
1. Focus your fire. Don't just let your ships attack on their own. If each ship targets one enemy at a time, that target goes down quicker, and that's the less damage your ships take. Remember, you don't want to create a huge fleet and find it was good for only one battle.
2. Don't spam seeker vessels or any ship of that equivalent. Your just wasting resources when you could send just one to see what your foe is made of. Remember, seeker vessels are not meant for battle.
3. The best way to deal with enemy spamming is to spam as well. Remember, each ship has some sort of weakness. Javelis frigates are equally matched, and can also take down battleships from a distance. However, you might want to attack first.
4. Make sure your fleet is versatile. If you have a fleet made only of assault ships, they'll be picked off by bomber squadrons and Javelis frigates. If your fleet is made only of Javelis frigates, you will have no protection from bomber squadrons.
5. Civilian research can also help in battle. I'm pretty sure no one would like to travel from planet 1 to planet 74 one at a time when there's a nice, fast wormhole system that can take you to planet 74 within seconds. That wormhole research can make the difference between utter destruction because of slow response time and swift victory with the element of surprise.
I hope this guide was of assistance. May you fight harder and be victorious.