OTOH, there's a famous example from the script of Charge of the Light Brigade, in which the screenwriter - when the big climactic scene came - simply wrote, "They charge."
In a screenplay, you can describe the battle in general terms, but punctuate it with important points. (e.g., "There is a huge melee, ork upon dwarf, elf upon man... Heads fly, blood spurts, weapons clash and clang and armor twists and dents. This continues until only a few combatants remain standing."
Then you can go on and talk about what some specific characters are doing...
Just an example off the top of my head.
It also depends who you're writing it for. If you're writing for yourself, go to town, write whatever you think will help. If you're writing it as a spec script, have mercy on the poor reader.