Crystal Barricade is a 0/4 blocker that holds off Wild Nacatl, and its hexproof means you can't be targeted by Lightning Bolt or Tribal Flames. It also prevents noncombat damage dealt to your other creatures, which blanks burn aimed at them, though it doesn't protect your noncreature permanents. Erode can destroy a problematic Scion of Draco or an attacking creature, letting its controller fetch a basic land but removing the immediate threat. Domain Zoo is a fast aggressive deck, so trimming Young Wolf and Carrion Feeder (which can't block) for more resilient defense helps you survive the early turns before your engine takes over. Keep Chord of Calling flexible to find answers or finishers as needed.