Now you have this nice pile of troops but how to use them is a mystery. To effectively attack and defend you have to understand how a battle goes, what happens when a march you've send arrives at a wall?
Step 1: Setting the range - distance between the armies - of the battlefield
The range is set by the attacking army. Standard range is 500, this is the minimum distance between the troops. This standard range can be enlarged by sending ranged troops with your march. Longbowmen for instance will add 1200 to this standard range. If you have weapons calibration maxed out at lvl 10 this will add 50% to the range bringing it up to 1800. So the distance between both armies if you have maxed out weapons calibration will be set at 500 + 1800 = 2300.
Step 2: Order of movement
The fastest troops will move first, from whichever army they might be. If the defender has faster spies then the attacker, they will move first. There is some debate on who gets to move first if same troops have same speed but most people assume it is the attackers force who gets to move first then. So, if attacker sends swift strike dragons, battle dragons and fire mirrors (bad combo by the way) and the defender has spies, battledragons and giants, the defenders spies will move first. Then attackers swift strike dragons, etc. Remember that research affects speed too.
Step 3: What targets what?
This seems to continuously change with each update kabam does.
In general, speed beats ranged, ranged troops beat melee and melee beats speed. So if someone is defending with a large number of dragons and vengeworms, you send ranged troops shielded by some melee troops. This is a generalized idea. The combinations that work change with each realm and the stage at which the realm is. Watch your alliances reports, talk to others and try analyze what happened. Best way to learn is try!
When defending, general rule is to send your speedtroups out. They die quick and uselessly. Now armory is out and people have gigantic armies battle dragons can actually act as a decent meatshield but this is advanced stuff. So when defending, in general, send you spies, armored transports, swift strike dragons, battle dragons, venge worms and stormdrakes out.
Step 4: battle language.
Like I did here, type everything out, is not usually how people talk battle. SSD are swift strike dragons, WC is weapons callibration. You might have heard about 'speedbumping' and 'rainbows'. The first attack method has become useless after last update. Rainbows is sending a mix of meat and ranged to do max impact damage. In some cases even sending speed troops with seems to work. after the last update has changed everything