Contradictory Instructor, Shell Master

:Sentinel(You may only have up to four cards with ":Sentinel" in a deck.) :[Choose a card from your hand, and discard it] When this unit is placed on from hand, you may pay the cost. If you do, choose one of your vanguards that is being attacked, and that unit cannot be hit until end of that battle. Then, if there is a card named "Contradictory Instructor, Shell Master" in your drop zone, choose a card from your damage zone, and turn it face up.