At first, the heavy combat at the heart of line is daunting, for any one. It seems like little more than chaos. Confusion and fast flowing battles muddle who is on your team and where you need to be. It takes time and experience to learn who is who and where death can find you.
To truly become a skilled line fighter, there are a number of thing you need to learn and work on. I'll break down each in more detail as I get the urge to write about them.
Many people are going to assume you have to be a great one-on-one fighter to survive on the line. It isn't wrong that good fighters will do well, but single combat skill doesn't directly translate to battlefield prowess. The greatest fighter will still die to backstabbing, outnumbering, and bad luck. Skill alone won't make you a great line fighter, but it can't hurt. Just remember, there is always someone better than you at one thing or another. Don't be intimidated by that fact, it just means you have people to learn from and people to teach. The best sword and board fighter may be just terrible with a two hander. Seek out the people that are better than you in one style or another, and fight them until you pick up a few tricks.
Keep that head on a swivel. New fighters tend to concentrate too much on the one target directly across from them, ignoring the person that's about to kill them. Look around, know where your friends and enemies are, know what support weapons are nearby. The more you know about your surroundings, the easier it will be to not only survive, but to bring down the enemy. Spot the gaps, in both lines, and use them to your advantage. Communicate what you see and help those around you with their awareness, all while listening for them to return the favor.
The Range Game
Can that spear stab you? Are you sure? What about that guy diagonal from you with the long sword? The better you can judge the range of different weapons, the easier it is to avoid them. Once you've learned them well enough, you can start playing the range game. Lure opponents into a false sense of security, kill them, and avoid retaliation; all thanks to knowing how far someone can reach. Always know your own range, and only engage targets inside of it. If you can't reach someone, why waste the energy on swinging at them?
Know Your Role
Your role can change from fight to fight, moment to moment, but always have some purpose to your actions and placement. Looking like you have a plan and know what you are doing can be just as effective as actually having one. Learn the basic positions, from the center to the flank and frontline to reserves. The center forms the core of the line, heavy armor and big shields, the wings are more mobile and form the bulk of fighters, flankers work to get around the enemy line, anchors hold each group together to form a cohesive line. All the while, leaders, shock troops, and reserves wait behind the line. Leaders keep an eye out for problems and redirect troops to fill in where needed, trying to keep their mind on the battle as a whole. Shock troops and reserves are somewhat interchangeable, both being held back until they are needed. The difference is largely in what their goal is. Shock troopers are often deployed to either heavy fighting or gaps in the enemy defenses, their individual skill seeing them through the fight. They are often deployed in an offensive role. Reserves are often a mix of experience and skill, used more often to fill in weak spots in the line or bring up support weapons where they are needed most, and are more defensive in nature.
This what wins or loses line fights. The heart of studying line battles is learning what causes and prevents decisive action. Smaller fights are often determined through attrition, but larger fights may be won through rolling a flank or punching a hole through the enemy line. Decisive action is often created by the some combination of effect from support weapons, flankers, and shock troops. Their combined efforts open gaps, weaken sections of the line, and put skilled fighters behind enemy lines. Gaps, kill pockets, and poor formations are all key topics to understand and look for. Often, the moments following some sort of decisive action, one side will suffer extreme casualties and likely be out of the fight. This is the climax of any line battle, everything afterwards is largely cleaning up stragglers.
Teamwork and Support
You won't get very far without teammates. Make sure you are working with other fighters. Swing at the same people they are fighting, block missile weapons heading their way, step between them and that spear that's trying to kill them. Communicate what's going on, where you are going, and when you need help. Got a spearman behind you? Call out targets while protecting him. Got a spear and have a shieldman in front of you? Tell him where you need him to be to help you. A small group working together can beat a much larger force that isn't.
I'm sure there are far more topics, but those should give me enough to elaborate on in the future.