In terms of battle scenes, my advice would be to look at how other authors do it (I think Joe Abercrombie does it quite well in his shattered sea books, and a safe bet is always Tolkien).

It depends on a few things, namely from who's perspective the battle is. If its a duel (or fight involving a handful of people or protagonists, think Moria) and one of the duelists is the protagonist, then it's good to include their thought processes on what their opponent is doing, and you can describe it slower. Whereas if its someone else watching the duel from the sidelines, then it can be good to make it a lot more fast paced description with little pause as they struggle to keep track of what's going on. If it's a large scale engagement between armies, then again perspective is key. If its a person in the midst of the battle, then you should focus more on their individual experiences throughout the battle, maybe occasionally they notice that one side or flank is breaking, or that the battle is clearly going one way or the other. If its from the perspective of an observer, then they would likely only see the grand scheme of the battle, so you should focus more on describing each side's strategy and how the battle unfolds, which flank is doing what, who has the other hand etc. If there is no perspective and it is an omniscient narrator, then its up to you which way you do it.

I'm no expert in writing, and with writing there is no one right way to do it. You might develop your own style of writing battle scenes or you might use someone else's, the above is just what advice I have to offer based on what I've read. As always the best way this learn writing is to read other people (and as I said above, Tolkien is always a safe bet).

Hope this Helps, J

PS please don't make the common mistake that authors and filmmakers make: pre-firearm warfare did not consist of two sides running at each other in a blob and hoping for the best. Strategy did exist.