This comes all out of experience and if anyone feels the urge to correct me please feel free at the end and maybe this will be 100% correct and get stickied.
Well there is a variety of ways that a certain fleet can come out of a battle alive, but you have to try and minimize losses, so you have the most profitable outcome of a battle. This is the main idea out of every battle otherwise you will run out of fleet and credit (to build more) very soon.
STARTING HINTS:
SHIELDS
As many of you noticed, advanced ships are equipped with shields of a certain number; Cruiser(2), Heavy Cruiser(4), etc etc. That means that a ship attacking any other ships, with less firepower, those ships shield will allow almost zero damage. Actually you do 1dmg every 100units that means you need like a million. Note that firepower and shields change according to research, so check other fleets stats before judging.
ARMOR
Armor is a very important part of research that any "base-busting" fleet should have advanced, since the debris is calculated as 2*(Armor Research)/100 of the Total Cost of the Ship. Remember, you really want the debris back!
PLASMA
There are 4 different units that use plasma technology. That makes Plasma research extremely useful since with one research you can better 4 different types of ships and researching Plasma at high levels will cost much less than researching any other type. That means by using cheaper ships you can really take out bigger and more expensive ships, for example at my research level, 3 Heavy Cruisers (costs 1500) can take out 1 Battleship (costs 2000).
FOR MUCH LATER:
PHOTON
For Dreadnoughts, Leviathans, and Planetary Rings I will get Photon technology as high as possible, since it is better that way so my defense is powerful and my fleet is more flexible because they can use Jump Gates (Death Stars can't use gates because (Disruptor), so I like Leviathans more.)
All other research do their part some more some less but this 4 are really important for an offensive fleet!
DEFENSIVE TIPS
In a war the most important thing is to make the occupation of your base very expensive for the invader. You can do this by building up defenses, Command Centers and some fleet. It is better to build fleets that you have higher research levels (like Destroyers, Cruisers, and Heavy Cruisers because of your high Plasma level you probably have by now) and thousands of Fighters that are very cheap (and you probably also have high level of laser by now).
You may think that fighters are useless, since they can't hit on shielded units, but they are not. They can be used as cannon fodder, so your other ships and defenses survive long enough to take out the enemies larger ships. So you must have many Fighters along ur main fleet. Don't forget this is a trap, though. Fighters are useless, if you dont have larger ships and/or defenses to do the real damage.
BOMBERS ARE NOT FOR DEFENSIVE FLEETS! (Any kind of bombers, especially Ion Bombers)
Also, it was said before that you need to have Command Centers! I can't say anything more than to boost your fleet power, since they boost a percentage each level, so that makes them that necessary(at least level 10).
THIS IS IMPORTANT! Its better to have larger defenses to kill out enemy fleet, than many different types of little defenses, mainly because they take more space than deal damage. If you insist on using Deflection Shields or Planetary Shield, be sure to recharge them after each attack. Use this:
-Defense Fleet against Enemy Smaller Ships
-Large Defenses against Enemy Larger Ships
I have a rule (which can be bent) but I never make more than four types of defenses on a planet. I just make the best four I have and when I get a better one I dismantle the less expensive one and build the new defense available. This is to avoid getting attacked with a "Wave Attack" technique, that I will explain later. I said this rule can be bent a little, since I currently have 2 Photon Turrets, 2 Disruptor Turrets, 2 Deflection Shields, and I am also building Planetary Shields. I will not take down Photon Turrets just yet...
Note: Shields are not useless (I'll say they are a must), since they have a lot of armor, thus a lot of health, that can be used to recharge rest of the turrets after each wave!
OFFENSIVE TIPS
I hope you didn't pass all above and come straight here. Again, there are a lot of ways to take a base, but its really hard to take out a base that the owner knows what he or she is doing. Although, in the case the owner is a newbie or noob here are some usefull tips.
TACTICS
DON'T send your Recyclers or Carriers to battle in a campaign! Use them for collecting debris and fleet transportation only. They are not Fighters.
NEVER send your fleet towards an enemy planet from long distances. I always send it to the nearest region, then to the nearest system, then to the nearest planet and then attack.
BASE BUSTING:
WAVE ATTACKS:
This is the most popular way to attack a base. this only works if the defending base has a lot of different types of defenses. It is ironic since the more types of defenses it has, the better for you. This is because after an attack, the health of defenses get distributed equally among all different types of defenses. That means if you have for example; 1 level Laser Turret and 1 level Disruptor Turret and you send 200 fighters in. After the attack, the Laser Turrets will have 0 health and Disruptor Turrets will have 4.99 health. That is what the Battle Report will show, but if you check out the base it will show the Laser Turrets have 2.5 out of 5 health and Disruptor Turrets have 2.5 out of 5 health! So, that means you destroyed 2.5 Disruptor Turrets with just 200 Fighters (well, numbers aren't exact)! In reality though, you may need to use Heavy Bombers to drop Photon Turret health and then in turn the Disruptor Turrets, but you get the point. Also, this theory doesn't really work on bases with Shields (Planetary or Deflection), but you can still use this theory to take out the defending fleet.
This is the same principle with other defenses, too, you send many lower type ships that can take out some of the lower defenses, then send some more to take out the rest, and finally the whole fleet to take out major defenses and occupy the base.
Example-
An enemy planet has;
1 level Laser Turret
1 level Missile Turret
1 level Plasma Turret
1 level Ion Turret
1 level Photon Turret
1 level Disruptor Turret
Phase 1
At first, you send lots of Fighters, 1k will do(much more if he has fleet or fighters at planet). This will take out all defenses up to Plasma Turrets and Probably Ion Turrets too, because you will have more Laser level research than he or she has in Ion level research.
Phase 2
Send in Heavy Bombers and take out Photon Turrets.
Phase 3
Send in your whole attacking Fleet and take over the planet!(Disruptor Turrets will now have around 2.25 health out of 5)
Phase 4
Send in Recyclers quickly and take the debris. As soon as debris is collected you may want to hide the Recyclers...if u can that is.
Note: This attack would have little effect to bases with Deflection or Planetary Shields because their defensive turrets can recharge between waves. Also you can use same technique on a defending fleet that has a lot of different kind of units, so u can take out minor fleet units to make major fleet units more vulnerable...
Example-
1000 Fighters
300 Heavy Bombers
200 Corvettes
100 Destroyers
50 Cruisers
20 Heavy Cruisers
10 Battleships
5 Dreadnoughts
1st wave = You can attack first by using some Fighters to take out Fighters, Heavy Bombers, Destroyers, and Corvettes.
2nd wave = Use Heavy Bombers to take out Cruisers and Heavy Cruisers.
3rd wave = Full Attack! Make sure you still have enough Fighters for the last attack to backup your main fleet.
FULL SCALE ATTACK:
Well, bad luck your foe is not a noob (has only a couple of strong defenses and fleet)... so who cares, we are bad, and determined to win! At this point, I send all I got in and take the base knowing that it will cost me to lose most of my fleet, but the debris, pillaging, occupation income, and personal satisfaction will compensate it for me!
Others will suggest that you should stick to some kind of wave attack, depending on the defending fleet on that planet. For example, if enemy has 1.5k Fighters, 200 Heavy Bombers, 300 Destroyers, 20 Heavy Cruisers, 10 Battleship, and so on. Send in first wave of 3-4k Fighters to take out non-shielded units and then send in the rest of the fleet.
HIT 'n' RUN:
Use this way of attack when you dont want to split your fleets or when you dont have enough fleet. This way you damage multiple bases economy for about a month and take some pillaging money. Also, hurting their defenses for 1-2 days making them vulnerable to other guild members to attack, again, and stay in occupation. For this type of attack, you need to have large numbers of Recyclers, so you can collect the debris really quick and then move to the next target.
OCCUPATIONS
A planet under occupation gives only 2/3 of the income to it's owner, while the other 1/3 goes to the occupator. Research, Production and Construction is reduced to 50% that's why it's good to occupy an enemy's fleet-producing base. Also, if there is a Jump Gate at the planet, it can't be used by anyone anymore.
If your planet is under occupation your first concern is to make it very difficult for the occupator to collect the debris. You can do that by making Fighters and/or bring some from other bases. The Fighters will target first low level ships like Recyclers and then move to bigger ships. That way you can take a lot of his Recyclers out.
If u are the occupator and you need to leave the Recyclers on a planet for a while, I suggest that you also leave a large number of Fighters too to take out any of the enemy's fighters that are produced or dropped off.
I suggest when your done with collecting debris try to hide your Recyclers or move them somewhere safe, if its possible.
FLEET BREAKDOWN
ATTACKING FLEET:
It is good to have many different types of ships, so the enemy's firepower can be spread along your ships. That way your largest ships take less damage. For example, 4k Fighters, 600 Corvettes, 400 Destroyers, 300 Heavy Bombers, 200 Cruisers, 100 Heavy Cruisers, 30 Battleships, and the necessary amount of Carriers is a good distribution. It is recommended that you have at least 100 Recyclers for every 100k fleet, but I rather build 200 Recyclers for a 100k fleet.
DEFENSIVE FLEET:
Pretty similar to Attacking Fleet Breakdown, but take out the Heavy Bombers and the Battleships. Both type of ships are actually only good for assaulting purposes. Fighters, Corvettes, Destroyers, Cruiser, and Heavy Cruisers are better to use as a defensive fleet, plus all higher units. That way you do a better damage to cost ratio.
Note: I HATE FRIGATES! I think they are useless, also Ion Bombers are good only if you have to take out some enemy Dreadnoughts and higher units.