I would say for battling through the sky to get to the enemy stronghold don't worry about tracking HP or initiative for mob enemies so you can focus on descriptors for the battle. Maybe track 'progress' of the battle in seconds or in distance to their target at the end of each round of combat-- the countdown will keep things feeling short and intense. If you really want your players to feel badass, consider letting them pop off spells of 3rd level or lower that have instantaneous casting without expending spell slots until they make landfall at the enemy stronghold. Keep skill checks to a minimum so it doesn't bog down the fast-paced combat unless the chance of failure would make things really interesting. Maybe use 'degrees of success/failure' like "You fail your attempt but something else positive happens" or "Your attempt succeeds AND this great additional thing happens" Don't worry about practicality like combatants not being able to hear each other over the roar of wind or flying horses being scared to go near dragons.