Ummmmm Graves?

I played a game with Zed, and I won lane heavily, getting 5+ kills and 2 turrets before 15 mins. Our top won lane with Fizz, getting a couple of kills but not much push (He was building tank). Our Rengar counterjungled Khazix and picked up a few kills there as well, and our botlane had 7 kills against their botlane's 3. Everything was looking great, an advantage in every lane, I was roaming, Rengar was ganking, Fizz was farming up. Then Graves picked up 2 kills in an ambush, and guess what happened? They grouped up and we thought "Sure we can win a teamfight". It was just Graves and Zilean trying to push down bot since the rest of their team decided to split up and try to push the sidelanes. Our entire team jumped in, Rengar and I went for Graves immediately as Fizz went in to tank some damage. Somehow with his insane armor stats (E), Graves managed to survive the full combo of a fed Rengar and fed Zed. MF ulted so he almost died but then he revived with full health because of Zilean. Graves W'd to smokescreen and thus we didn't know where Zilean put his bombs, and he ended up getting a 2 second stun on 5 people, which magically dealt upwards of a thousand damage to each person on our team, despite having only 350 ap (Insane scaling or something?). Graves ults, he gets a triple, and oneshots our Fizz and Bard with a single aoe auto for the penta. The game proceeded to consist of Graves wrecking our whole team and surviving everything because armor and Zilean, and winning. Somehow he went from 2/5 to a 2v5 pentakill against a team of fed people. Is there any counter to his massive armor? Don't say "build armor pen" because I had youmuus, duskblade and black cleaver.

tldr; 5v2, Rengar and Zed go for Graves, he somehow survives full combo, Zilean revives him at full hp and bombs our whole team with a 2 second stun and 1000+ damage, Graves ults, we all die. How to counter his free armor?