Hail to the King: Death Bat Walkthrough Nightmare Bonus Round
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The bonus levels offer huge gold rewards and are achieved by unlocking all 3 bat wings in each level. This bonus round paid out over 500 coins on a bad run, and can be farmed over and over for 300+ coins per run.

Avenged Sevenfold own vocalist has a tutorial for level as well. Here's the link: https://www.youtube.com/watch?v=qN0oE-TpRzc View SandSeven7 Channel