Well, given that your average November novel is 50k words, 20k to 30k is a little excessive. You can’t keep a single fight going for that long, without the reader wearing out, though you could comfortably stuff an entire war in that word count.
Unless the weapons are magical, the guns will win out every time. I keep saying this, but there’s a reason we no longer take longswords into battle. If these are enchanted armaments of heaven and hell, then you’re the one setting the ground rules for what they’re actually capable of.
If you’re willing to go back to the drawing board, I’d recommend digging up used copies of In Nomine. It was an RPG setting in the 90s, with angels and demons engaged in a shadow war for the universe. If you can find it, White Wolf’s Demon: The Fallen might also be a good, if bleak, source of inspiration.
If you want to look at a setting with mass warfare, particularly where guns and melee weapons do mix, I’d recommend Warhammer 40k. I usually recommend the Ciaphas Cain novels as the best 40k entry point, but in your case, one of the Grey Knights novels might be closer to what you’re trying to do.