Repose and Sleep have the same success rate (100% vs targets vulnerable to it).

The big turtle is not vulnerable to stuns while it is casting its Stomp, a very uncommon form of invulnerability. Leviathan Extreme's Wave Fang adds, the casters, also have this immunity while casting "Dreadstorm" which leaves behind a puddle that causes hysteria when stepped into, and is the reason groups normally tried to chain stun them to 0% as they begin to chain cast Dreadstorm.

Repose is as useful as they give us places to use it. Knowing about it on the turtle is cool, thank you! I hadn't tried myself because I've mostly been avoiding playing healer in duties at all, even in fully made parties.

Last time Repose saw a good use like that was Amanesis Anyder on the Sahagin that would summon adds from the crystals after the 2nd boss; sadly they made them immune to Sleep after a short while.