Blue Storm Shield, Homerus

: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, if you have a vanguard with "Blue Storm" in its card name, you may pay the cost. If you do, choose up to one of your units that is being attacked, and that unit cannot be hit until end of that battle. Wave-3rd time or more (This ability is active during the specified battles of each turn): 1 At the end of the battle that this unit boosted ([Boost]), if you have a vanguard with "Blue Storm" in its card name, you may pay the cost. If you do, return this unit to your hand.