Did a solo run of each chapter on heroic, which took me about 25 minutes each. Got enough experience to build out two loadouts. Then did them again with a co-op buddy, then a group of three of us on legendary. We finally discovered our team groove and burned through each chapter in about 12-13 minutes of seriously intense firefights - Promethean Knights were made for legendary - their mobility seems to be intensified, and their melee lethality is through the roof. They are properly dangerous and keep you on your toes and working as a team. Crawler and watcher packs also teach you to move in formations to avoid getting surrounded.
And to think we're getting a total of 50 of these missions for season one. I am loving this.
BTW - A lifesaver for me when soloing these missions has been Hologram. Its got too many uses both on offense and defense. So if you're hitting a wall, its something to consider. Also, while thrust pack doesn't seem to offer all that much mobility, it has saved my ass at least a dozen times already. I'm having a hard time using any other AA at the moment.