Caspiel will be about as useful as Zephyr, since there's a limit to how quickly you can stack the -5% ATK down debuffs in a real fight before they start wearing off. But if you stack Caspiel with Zephyr, you'll be able to mitigate damage up to 85%, making your team nearly invulnerable (except to debuffs or if negated by powerful attack up buffs, which are common).