Headcanon that both of them are equally matched during sparring/training.
Leon has a little bit more economy and structure, because its hard to shake the habit as he's been fighting that way since he was a child/squire.
Lancelot has less structure and a bit more flow, like muted down version of Gwaine's fighting style. There's structure to his strikes, but there's also the odd little flair or trick that catches Leon off guard, it comes from being a wandering Knight and having a different range of opponents.
Whenever they fight, they eye-flirt too. Like, full on facial expressions eye-flirt. It's honestly disgusting.
Their sparring is mesmerising to watch, and the added romantic tension just makes it better.
Merlin annoys Lancelot about it more than he annoyed Arthur about Gwen in cannon. It gets so bad that at some point, Kilgarrah asks Lancelot how his 'mate' is faring.
Arthur is the last to notice, obviously, but you can bet he slides a sly remark in every once in a while when he and Leon are going over inventory of weapons. Leon thinks he's kidding, until Arthur forgets the two of them are not actually together yet.
Arthur, completely obliviously: So when are you and Lancelot celebrating your anniversary? Isn't it some time this week?
Leon: ...
Arthur: ...🤨
Leon: ...my- my lord, Sir Lancelot and I are simply good friends.
Arthur: ...Wait, whAT? 😲
When they finally get together, it's because Gwaine got them drunk and they kissed in the tavern (after everyone told him it wouldn't work).
Nobody was surprised.
Not even Leon and Lancelot.
OI
MERLIN FANDOM
I KNOW YOU'RE AWAKE
I had a thought and I need other people's thoughts:
Leon/Lancelot?
Sincerely,
A Merwaine shipper