I just started running this dungeon tonight for the Operation Midnight event.
Best Time: 0:26
Avg. 0:33
My Stretch Goal Team:
>Eagles Dare Ed- Ragnarok/Cosmic (Death Mark Skill)
>A. Axis Bomber Boy- Starfire

You just need 2 characters.
W.E.D. Ed becomes a multi hitter w/ Death Mark Skill, plus Ragnarok inflicts true damage w/ every action.
The Starfires do the rest. Enemies attacking on Waves 3 & 5 pretty much kill themselves.
Mistakes would be using echo cosmic on single hitter Ed, and using Celestial or Alchemy (w/ invincibility) along with Starfires, cuz no damage prevents the enemy from triggering Starfires. These things would prolong the battle, and considering you get Explosive frags according to the number of battles, Time & Quantity of runs is the name of the game. I'm sure alot of you know this, but I state the obvious for some of the newer players.
Good Luck Troopers!