Peek-a-boo has you kill a shrouded enemy while they are cloaked, so naturally the first order of business is to find a shrouded enemy. There are two solid places to find them; at all of the repowering stations on the map (A large amount are in the Dune Sea, on the bottom left), and during the second story mission for Marshall called "The Signal". During this mission, you'll go to a base teeming with shrouded. Once there, simply wait for one to go invisible and blast them (rocket launcher
is your best bet, but of course you can do it with any weapon). Note:
From my personal experience, it seems like some shrouded are at a disposition to go invisible more often, so if you're having trouble getting one to go invisible, just try going for a different one.
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