The key here is that Byrke, Long Ear of the Law doesn't have to be the creature attacking, so you can get double counters on two attackers the turn Burke enters, and it gets out of hand on subsequent turns
Rammas Echor, Ancient Shield Rammas Echor, Ancient Shield has something I really enjoy: it cares more about what you do rather than depending on your opponents' moves
Erayo's Essence Legendary Enchantment Whenever an opponent casts their first spell each turn, counter that spell
You must still follow any timing restrictions and permissions for the permanent spell you cast from exile
A three mana enter-the-battlefield tutor is an incredibly powerful effect, especially if you have a way to trigger it multiple times
A player loses the game after having been dealt 21 combat damage from any one of them, not from both of them combined (although your Background wont usually be a creature anyway)