Mardu Skullhunter enters the battlefield tapped.
Raid — When Mardu Skullhunter enters the battlefield, if you attacked with a creature this turn, target opponent discards a card.
9/29/2017 Raid abilities care only that you attacked with a creature. It doesn’t matter how many creatures you attacked with, or which opponent or planeswalker controlled by an opponent those creatures attacked.
9/29/2017 Raid abilities evaluate the entire turn to see if you attacked with a creature. That creature doesn’t have to still be on the battlefield. Similarly, the player or planeswalker it attacked doesn’t have to still be in the game or on the battlefield, respectively.