Maybe you're thinking of Day of the Dove where the creature gained strength the more they fought.

Yes, I think this was the one. So, no, I wasn't imagining it.

Actually, I don't think Day of the Dove is the one you were thinking of. Day of the Dove was nothing like that scene in Fifth Element.

When he said "the creature gained strength the more they fought" he didn't mean the more they fought the creature. That episode was the one where the creature fed off of the energy of them fighting amongst themselves. It was the one where there were a bunch of Klingons on the enterprise (perhaps put there by the creature if I recall correctly). They defeated the creature by getting wise to its scheme and laughing it off the ship.