Countering undying

[Wally]

There is a simple way to counter undying. You can respond to the undying trigger on the stack by exiling it with cards like  {Deathrite Shaman},  {Scavenging Ooze},  {Beckon Apparition}, etc.

By the time the undying trigger looks to put the creature back on the field, it's already gone.

[Agrus Kos, Enforcer of Truth]

Yeah at the prerelease two brothers each made GB Aggro decks and each managed to pull a {Scavenging Ooze}, let's just say it was hilarious and no one had a graveyard.