It should be quite obvious that you need to complete the Main Mission "Ground Zero" in order to unlock the Side Ops. "Jamais Vu" is unlocked after completing this achievement:
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It doesn't matter which difficulty you do these on. It can also help to have received an "A" rank on the mission before going for your "S" rank since it will allow for some extra equipment to be used. Some of the Side-Ops can be difficult but these videos I made while livestreaming can provide some useful tricks to make them quite easy.
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Main Op : Ground Zero on Hard (33:25)
http://www.twitch.tv/gamemaster136/c/3985043Pretty straightforward. You can take your time and make mistakes as you can make up for them by rescuing prisoners.
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Side Op : Eliminate The Renegade Threat on Hard (7:06)
http://www.twitch.tv/gamemaster136/c/3977787Shows where the sniper rifle is if you don't already have it and how to score some free extra points by extracting one of the targets.
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Side Op : Intel Operative Rescue on Hard (11:35)
http://www.twitch.tv/gamemaster136/c/3982161Just don't kill anyone okay.
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Side Op : Classified Intel Acquisition on Hard (5:47)
http://www.twitch.tv/gamemaster136/c/3977833Did you ever find a good use for that LAV? Well here's how to make good use of it.
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Side Op : Destroy The Anti-Air Emplacements on Hard (Hard Way | Without Unlocked Rocket Launcher) (11:27)
http://www.twitch.tv/gamemaster136/c/3977857It is recommending to get an "A" rank on this so you can do the easy way.
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Side Op : Destroy The Anti-Air Emplacements on Hard (Easy Way | With Unlocked Rocket Launcher) (5:11)
http://www.twitch.tv/gamemaster136/c/3982229Makes an annoying mission quite easy.
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Side Op : Jamais Vu on Hard (15:55)
http://www.twitch.tv/gamemaster136/c/3985028You thought the end was hard? You're not making great use of what's available to you then.
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Hopefully these help. First guide so I hope everything is sufficient.