During an intergalactic war a battle fleet wins a decisive, but costly battle leaving an inexperienced commander to rally them when they are attacked by a gigantic space-faring beast that begins picking ships off one-by-one.

“Winning a costly intergalactic war, an amateur commander leads an unprecedented assault against an alien that is preying on the fleets ships one-by-one.”

Be concise and cutthroat with your words. This concept has potential and with a few more tries could have a logline to match. 😀