This my guide on battle theme music after listening to quite a few of them.
Standard/Boss Battle Theme Tips
1) Make sure that the theme is badass and memorable.
2) If you're going to do lyrics, make sure that they make sense in context.
3) If the boss has a cultural theme associated with it, use instruments to reflect that.
Final Battle Theme Tips
1) Make sure that the theme is more badass and memorable then normal battle theme.
2) Make sure that at least part of the theme sounds dark and/or intimidating, to emphasize how powerful the enemy is.
3) You don't have to, but you can make at least part of the theme sound somber just to emphasize the horrors of war and the hardships everyone went through on your journey just to get to the final boss.
4) If you're going to do lyrics, make sure it makes sense in context.
5) You don't have to, but to make the theme even more unique and memorable try using an instrument no one would expect to hear in a final boss theme, such as a kazoo, bagpipes, a tuba, an accordian, and even an harmonica. Bonus points if you make it sound scary.
4) If the final boss has a cultural theme associated with it, use instruments to reflect that.