Also correct me if I'm wrong but once a mage uses ms, you can't health sap with redemption let alone drain. If you use those skills on a shielded mage, it just does damage to their mana pool without any regeneration.

This is one of the big issues for rhino when fighting a mage since rhino depends on getting their hp back. The same is true when a mage casts drain on a shielded mage (I think?).