A really good author for making large scale battles interesting is G.A. Henty. He wrote historical fiction during the late 1800's, retelling nearly all the great battles of history in over 120 novels. His method of choice was usually to tell the battles from a bird's eye view, detailing troop movements, then go back and tell what his main character saw of the battle and how it affected him. He was also a master strategist, (if you want any information on guerilla warfare throughout the ages, his books are the place to go).
For me personally, I haven't written a battle in some time, so I'm not sure how my current writing style will affect that, but I would echo what some of the others have mentioned with couriers, this wing calls for aid, this wing sends word it is advancing. Another factor to consider is the ground. If there is any height involved, someone might have a good view of at least most of the battlefield. On the other hand, what kind of technology is being used? Dragons/phoenixes would get the literal bird's-eye view, while dragon's fire/fire magic might create smoke dense enough to obscure everything. (Dust from the ground could also do this).
Tactics also play a role, someone laying in ambush might only hear what is going on until his signal comes, a dragon air strike would provide an excellent overview of what is happening.
The last factor to consider is who your pov character is in relation to the battle. A general might be more aware of the battle as a whole, a simple foot soldier only of what's directly in front of him. A calvary/dragon rider would see more of the battle, but would be less in direct combat, whereas a archer/siege weapon worker might have more time to watch the battle as it progressed after their projectiles were exhausted or rendered useless by troop movements.
And finally, the most important thing to include is the feel of the battle. No matter how detailed the troop movements, the thoughts of the combatants or the originality of the tactics, it all comes down to what the reader comes away with afterward. Did they sense the chaos, smell the dust, see the price paid by those who gave their lives, hear the whine of arrows, the clash of swords, the roar of beasts? Did they cringe as the flights of arrows came whining down? Did they cheer with the embattled soldiers as the reinforcements came thundering in?
Just some thoughts to run on, hope it helped someone!