To unlock this achievement you MUST, in MASTER difficulty, beat Treacherous Waters (Level 7) with 100%.
Here are the objectives you need to complete to get 100%:
1. Rescue 4 Villagers.
(NOTE: There are 2 Coins in this level. If you have already collected them before in a previous play through you do not need to collect them for a 100% score, so you can skip them if you wish).
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Villagers
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Finding these four villagers is super easy. You will find the first 3 villagers near the start of the level and you will find the last villager in the middle island after the cut scene.
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Efficiencies
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At this point, if you have not unlocked the "Untouchable" achievement you can use a bug that I found to unlock it with little or no effort at all. Check out the guide I wrote for more information.