When you purchase booster packs, you have the chance to receive the Revive consumable cards. These are used for Garden Ops. When you get downed in Garden Ops, you can either wait for a teammate to revive you, or you can use a revive card. Now one thing to note, you only get 3 revives per game in Garden Ops, so don't just rush to use these simply to pop the cheevo. There will be times in the later waves where you'll be wishing you hadn't wasted them in the first round when your teammate was able to get you.
I hope this guide helps and happy hunting!
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