To decipher any cipher try the following:
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Use a cipher statistic tool to gather information about the cipher used (You can use this one http://web.archive.org/web/20120624074941/http://home.comcast.net/~acabion/refscore.html).
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Google a decypher tool based on the statistics.
As a tip for this challenge, I had to find an accurate tool to subdivide the ciphertext into a number of subtexts to gather the flag.
Also remember to submit the flag all in Upper Case and with the exclamation mark.