Biggest enemy and/or battle you’ve encountered?

[Frost]

I don't know if it would count because we never fought the entire force in one go.

Many years ago I was in a Traveller campaign where the PC's were officers on an imperial warship. After a misjump their single very elderly cruiser having found it's self behind a Solomani advance ended up with about two squadrons worth of ships (16-20) trying to run it down.

They were still trying to run it, or rather the surviving crew, down a year and at least six lost ships later when the waring parties blundered into the main imperial counter attack.