Warfare and fantasy go hand in hand. There is something visceral and exhilarating about medieval-style combat…although I personally would never want to be in one. It may be cool to watch elves, orcs, and horsemen fight each other, but in reality, it was brutal, bloody, and no fun at all. Still, that doesn’t stop writers, including myself, from crafting massive campaigns and emotionally charged duels. What can I say? Humans love to fight.
Now, please understand that I am not a soldier. Everything I know about war I have picked up from reading, watching movies, listening to my dad lecture on history, and plain common sense. If you want to learn what should and should not be done in war, I recommend reading a lot of history books. Human history is littered with good and bad generals, close calls, narrow escapes, massacres, ambushes, traps, intrigues, bad weather, advantageous terrain, underdogs, overlords, battles that went the way they were supposed to and many that did not. History is the best teacher. I also highly recommend keeping a copy of Brassey’s Encyclopedia of Land Forces and Warfare beside your desk. Its focus is on modern armies, but much of the tactics, concern with morale, supplies, terrain, weather, etc. can be applied to old-style armies. And I’m sure there are plenty of other books at the library covering most efficient ways of killing people.
Adding magic and fantasy creatures into the mix can be a little tricky because, obviously, such things aren’t a factor on Earth, so there isn’t an easy answer about how wizards or giants can affect the course of a battle. That’s where your own creativity and judgment come in.