Is it safe to assume that these two big groups are armies? One surefire way to get a battle like this right is to read things with a large scale battle scene. Lots of Arthurian legends (especially near the end of King Arthur’s reign) have large scale battles. Also Greek myths: Iliad, Aeneid, Odyssey have a lot of army battles. The Greek myths would often do this thing called aristeia which was the culmination of a hero’s life. Aristeia was the moment of a big battle where the hero’s life either came to its glorious end or the hero overcame their biggest enemy. Think of it as a Greek hero boss battle. So to sum up, large scale battles rage on around the main characters as they fulfill their heroic destiny is the way it’s usually done. However, ultimately it is your story. If you just want to describe the cacophonous sounds of war as hundreds of soldiers rush each other then, hey, that’s all you.