I used these two cards, but there are so many options to choose from: (There may eventually be a unit with both battle skills, but as of this guide - none exist) So your deck will need to include any number of units with Valor and something that gives a unit Overwhelm - weapon attachment or spell. The counter to that is the battle skill 'Overwhelm' - which allows excess damage to go through. Any attacker that is blocked will get +1/+1 for each blocker - but the nature of blocking is all damage is prevented from passing through and therefore the AI will only sacrifice one unit.

The 'Valor' ability is a battle skill available on multiple units. Just control the board by removing its units. This will keep the AI from attacking and you can take as many turns as you need. There are multiple ways to do this, but ultimately you will want enough units with a strength/health that can prevent damage without dying. Those familiar with the game will understand that you can create stalemate against the AI. The achievement pops immediately when lethal damage is applied and the match ends. The achievement can be earned in single player - Gauntlet.

Eternal card game gauntlet