Honestly, it's the grit in the details that sells a training scene for me, not the flashy stuff. You can tell when an author's actually practiced something, or at least done their homework. I'm thinking of something like 'The Way of Kings' where Kaladin's progression is just endless, brutal repetition—hauling a bridge, learning stances until his muscles scream. It's not about a secret technique; it's about the character's relationship with exhaustion and frustration.
That's what builds a real sword-wielder, in my view. The moments where they fail, where the blade feels wrong in their hands, where their old habits creep in and get them smacked by a training post. It makes the eventual payoff, that first clean strike or parry, feel earned instead of handed out by a plot coupon.