Armor naturally reduces the rate that attacks go through Granite Shield, so Brutal Strikes will strengthen the shield against physical damage.
Despite his abilities scaling on Armor, some games require Malphite to get Magic Resist. When those games occur, try getting Aegis of the Legion, Mercury's Treads, and Guardian Angel.
If you're a physical damage character stay behind your allies when fighting Malphite. Ground Slam can significantly reduce your damage output.
Malphite is one of a couple tanks who are capable of jungling. Watch out if he gets Smite.