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After you escape from Majestic 12 facility (actually UNATCO HQ) talk to James Reyes (the doctor) and ask him to be your spy inside UNATCO. Once you´re in Paris you´ll meet him again at building 19, the cafe. This time he will tell you Gunther´s killphrase. This is certainly the easiest way to defeat him, as JC states..."sticks and stones".