Here are some techniques for creating powerful, exciting, realistic battle scenes.
The biggest challenge in writing a battle scene is the point of view. To make the experience exciting and moving, it's best to stick to the perspective of a single fighter. However, the individual soldier can't see what goes on a few feet from him, let alone what's happening at the other end of the battlefield or how the sun dyes the horizon bloody red.
Here's a possible solution: Show the terrain before the fight begins, and have the general give a pep talk explaining the overall strategy. Once the fighting is over, show the battlefield and have your point of view character talk with his comrades about the implications.
Do you want to involve the reader's emotions? Stack the odds against your heroes. The readers' natural sympathies lie with the smaller army. The greater you can make the numerical difference, the better. The evil overlord's army is bigger than the hero's, and it is much better equipped, too.
Have you heard of the battle of Thermopylae (480 BC), when three hundred Spartans defended Greece against thousands of invading Persians? The Spartans knew they were going to die, and fought anyway, to gain time for their homeland to prepare further defences. Since then, thousands of battles have been fought - and forgotten. Thermopylae is remembered. The story has been retold in many novels, non-fiction books, and films. The incredible bravery against overwhelming odds still rouses audiences' emotions. When writing your own battle scenes, use Thermopylae as your inspiration.