Well, it depends
heavily on the scale of the battle scene and what part of it that the MC is supposed to experience. A general will have to think and face conflicts (both martial and narrative) on an entirely different level from a warrior on the very first rank of the battle-line. And a general whose idea of leadership is charging at the head of his troops will experience things
very differently from one who prefers to stand back and retain control over the whole force at her disposal.
I agree with Dominic that it would be totally unrealistic (not to mention no fun to read) when a character with no combat experience or military education at all suddenly becomes a military genius who knows every little bit of military terminology; but I beg to differ in the matter of
Wheel of Time, because most of the battles in that series have a strikingly modern feel that really threw me out of the story. Maybe that's just because I've read way too much of Caesar and Xenophon and the like--their works are so vivid that they spoil my enjoyment of the vast majority of fantasy battle scenes I read.
Tolkien's battle scenes in
Lord of the Rings work for me because they remind me so much of
Beowulf, the
Battle of Maldon, and the Icelandic sagas--but then he was a professor with a very intimate knowledge of such literary works, and we surely can't expect every fantasy writer to devote so much singleminded obsession to his/her project as Tolkien did.
One good way to get a grip on the techniques for writing battle scenes is to diversify you reading beyond the fantasy genre. Bernard Cornwell's
Sharpe series is a good example to emulate--he has a good hand at building up the tension and delivering the release in a satisfying climax. His description of Sharpe's swordsmanship and of the (outdated) line vs. column logic in the major battles make me cringe, but it does not dissuade me from reading on as much as Jordan's battles do.
Another idea is to ransack the local library for translations of...well, old stuff. The
Iliad is still the bloodiest work of literature I've ever read, and it's still my first choice to read when I'm in the mood to kill something. Xenophon's
Anabasis is a first-hand account of a celebrated military retreat, as full of diplomacy and politics as it is of battles and skirmishes, not to mention that it contains many personal anecdotes that can help you understand how to give a personal touch to narratives of war.
I've also written some amateur essays on pre-modern warfare specifically for the use of fantasy writers, but I don't know which one of them I should pimp out to you without more specific information about what kind of situation you actually have in mind.
