The main objective of this is to improve combat skills and secondly make our cities as prosperous as possible.

Some of the important things that you can do to make your combat engagements more productive.

1. Level your combat multipliers as quickly as possible ( CoA, Zeus and CDM). As you have probably noticed in engagements on the Continent Map, Guild Expeditions, GVG and PVP battles, the units you will engage will have combat boosts of their own.
If your boost is less than your enemies you will most likely suffer high losses.
The higher your boost the less losses you will have.
2. Study the attack and defense bonuses of your individual units. It is important to select units that have capabilities suitable for their intended purpose on your Offensive or Defensive Armies. ( more on this later )
3. In most cases it is desirable to use 1 to 2 units in combination with rouges on your attacking army allowing you to take advantage of the rogues ability to shake off the first damage as they change to the one of the units you have chosen.

The ratio of units to rogues is dependent on where you are fighting at.
Fighting on the Continent map is more difficult because the game AI is more aggressive and will tend to engage your units first requiring you to use more than one unit in combination with your rogues to avoid having your unit killed which renders your remaining rogues ineffective especially in two wave armies.

Defensive armies work off different multipliers that are dependent on several items you may wish to have in your city. Watchfires and Ritual Flames (4% boost from WF’s, 8% boost from Ritual Flames in defense only × the number of WF’s/RF’s in your city at the time). Two Great Buildings, Saint Basil’s Cathedral and Deal Castle will increase both the attack and defense bonus of your units while defending your city. Many like to use these GB’s to add to their defensive prowess, while many others prefer to employ the space-saving ability of WF’s/RF’s to obtain a strong home defense. (See more at What Great Buildings are Right for ME?)

It is not desirable to use rogues in your defensive army. 1-2 can prove frustrating for your attacker, any more than that and they will target your units first, then poof go your rogues.
Each era, you have 5 military units to fight with. Each unit has an advantage over another unit. When fighting on the continent map or in GE, you get to see the units you will be fighting. At this point, you need to adjust your attacking army to best match up with the army you will be fighting. From Bronze Age through Progressive Era, here are the 5 units and which units they match up against the best.

Artillery units are good against Heavy units
Light units are good against Fast units
Heavy units are good against Light units
Range units are good against Heavy units
Fast units are good against Ranged units

From Progressive forward, which units are best against which changes up with the ages. By this point in the game, you’ll have a pretty good grasp of how this works. Practice in GE to see which units work best for you and see how they attack other units. GE is a great place to test your skills and learn.

Also, if the units you are attacking do not have Artillery or Ranged, let them come to you. Don’t race out and take the first damage. Skip your turn and let them come towards you and get within range. Always better to deliver the first hit than get hit and be damaged. As a unit gets damaged, it’s attack is less effective. Also, if you are winning a battle and you have a unit or two with heavy damage, try to move it out of range from the attacking army to save that unit from being killed. Be mindful of terrain as well! Most units have a defensive bonus based on what type of terrain they are on (plains, hills, trees, etc). Some units even have special abilities, like hiding in trees, where they cannot be hit unless the enemy in right next to them, or if the enemy has the ability to ignore stealth. Opening your Army Management Screen and mousing over each unit will give you an overview of their abilities. The better you know your troops, the better you can make them work for you!