I'm gonna share some of my experiences on how to play a battle properly. First up, deployment of units. Under normal circumstances, I would recommend deploying your lowest level unit as your first set since they need the xp the most. Depending on the unit level, it can be either cycled(reinforce and then retreat units without engaging the enemy) or just sent into the enemy to take a few kills without coming back to the portal in one piece. Either way, this applies for units below level 25. Next, here's something that I practice, the levels of the units I deploy usually increase in level with each deployment. That means I deploy my lowest level units first, then deploy my higher level units later in the game. As I was saying, units below level 25 can be cycled, where units above level 25 will need to stay in battle for at least 2 minutes while engaging the enemy and taking a few kills before retreating the units to get some decent XP.
There are a few ways to go about playing battles. One of them would be going through a routine based deployment, another would be deploying to respond to situation, the last would be playing as a distraction for the enemy. I'll try elaborate on these 3 aspects of gameplay.
What I mean by this would be going through a fixed pattern of deploying your units. Example being for every battle your have the same units deployed in the same manner and in the same sequence. Even the timing for retreating and reinforcing your units will be similar. This style of gameplay is quite common among the fast levelers in SG. Units are reinforced into the battle for a fixed amount of time before they are retreated to bring in the next set. This is a fairly predictable style of gameplay and it can be easily exploited by the enemy when they figure out your sequence. But it still works anyway.
The players playing this way usually do not bother about getting xp for their units or leveling up(EMPing would be a great example). Their main objective is to act as a distraction for the enemy and hence buying time for the our factionmates in the battle. This way of playing is extremely unpredictable but it does not help when too many people play like that, instead it would benefit the enemy.
Last but not least, deploying to respond to battle situation. This is definitely the most effective way to approach battles however it requires some concentration and effort. The phrase 'deploying to respond' means that you deploy your units according to what is needed in the battle. Let's say you are reinforcing a map that is being FPed by the enemy,under such circumstances, it is recommended to bring your fastest defender and it should also be able to inflict high damage. Some recommended units would be ghasts,war pigeons,pelicans. Arbalests are an exception because they have range.
Other situations include:
Enemy Air Mob: Bring GA (Ground to Air) units (manticores, hydras, liches, GA spirits, ballistas) or AA (Air to Air) units (hawk, owl, phoenixes, condor)
Enemy Arbalest/Spectres/Ballista/Redeye discovered:
Use fast GG (Ground to Ground) cloaked units (pegasus, ghast) or AG (Air to Ground) units (pelicans, versatile phoenixes, war pigeons) to eliminate these targets as soon as possible.