And how it works in-game it says you need to have 300 resistance in total to open it, and some of you may or may not be aware that their current adds some resistance as well which it dosen't count towards the total.

Here's how you can unlock it and to make sure you have the total for one.

1.Go to your sylph page and use the rest button to disable the sylph.
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2.Go to your battle protection system, and here's an example of how it should look without your sylph on you.
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And you can count if you have a total of 300 that is the real reduction without a sylph on you.
3.Now if you have more than 300 in total, go to your Battle Protection panel and click on the shield to unlock it.
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Hope this helps, if you have any questions please feel free to post