Although a focus sash prevents the user from being OHKOed and leave pokemon with 1 HP, it will be used after the first hit and Shedinja will be vulnerable to the next hit. No other item will make shedinja invincible either. The game would be pointless if it were possible to have an indestructable pokemon, and therefore no item, ability, ect making shedinja- or any other pokemon- invincible will be/ has been created.
