For Jason, all you need to do is parry once, throw a Coin in the air, and start CoinJuggling! Doing this can kill him in less than 10 seconds.

For Captain, however, you CoinJuggle 3 seconds after this cutscene starts, so the damage stacks carry over to when his boss bar appears, letting you get him to about half or quarter health. Its freestyle from here.

For Vagabond, do the same as Captain, then get him disarmed, and start CoinJuggling!

Your welcome.