Doomsday is a fast combo deck that wins through Thassa's Oracle after constructing a tiny library, typically protected by Teferi, Time Raveler and Force of Will. Gaddock Teeg is your single best hate piece — it shuts off Doomsday (CMC 4), The One Ring (CMC 4), and Force of Will simultaneously, so finding it via Green Sun's Zenith should be a top priority. Thoughtseize is critical here for stripping Doomsday, Force of Will, or Teferi before they can establish their lock. Endurance and Faerie Macabre serve as graveyard disruption and can answer Thassa's Oracle piles mid-resolution, so keep mana open after resolving your threats. Craterhoof Behemoth, Springheart Nantuko, and Hogaak are win-more pieces that cost too much mana or require too many board resources against a deck that can kill on turn two or three.