@arzi I prefer a slightly different method:
Put the player up against a near-impossible fight. Don't force them to lose, force them to grind away 10% of the enemy's health, unprepared. Make them feel like it's a slog, but they're making slow progress. Once they do, give them a reprieve by having them lose by cutscene and then give them the ability.
That way, you continue to build advancement on top of accomplishment.
Metal Gear Rising: Revengeance used this technique to good effect to reveal its Ripper Mode.
@LouisIngenthron Yeah, I'm starting to lean on doing it roughly this way.