Brink

This gentleman on YouTube details how to get it solo very easily indeed. Worked first time for me. This isn't my video and I take no credit whatsoever for it.

This achievement asks you to reveal an enemy Operative who is in disguise. In order to do this, you simply fire upon a disguised operative until he is incapacitated and he will instantly shift back to his normal form. You can also melee him, baseball slide into him, or hit him with grenade splash, as long as he's an impostor (and not actually a team-mate) he'll be knocked to the ground by these actions.*

(*Note - In Advanced settings, with Friendly Fire at the highest settings, I believe you can knock your allies down with baseball slides/melee attacks, so these may not be as useful in identifying spies. More just annoying team-mates, really.)

Operatives in disguise are fittingly difficult to spot. The easiest way to spot one is to be an operative with Homing Beacon - this will outline enemies in red, even if they are disguised. Alternatively, if he makes the mistake of trying to impersonate you - this can lead to some comical online moments! (try impersonating someone and following them around next time you're bored - see how long it takes them to notice their doppleganger shadowing them =P )
The only other way to legitimately identify an enemy Operative in disguise is to be vigilant for odd behaviour - disguised players can only move and SMART. Be on the lookout for so called 'allied' players acting strangely:
~ Hanging around near enemies without being fired on (His teammates will see his real name following him, they won't attempt to kill him)
~ Running against the flow of battle. (Unless he's being chased by enemies and bullets, there's no reason for your medics and soldiers to be running past you towards your spawn points)
~ Not firing. (Already mentioned above, but it really should be watched for, disguised players cannot fire. I'm not encouraging friendly fire, but a few speculative rounds into an ally who's avoiding combat will reveal his true alliegance.)
~ Finally, be vigilant for duplicate Gamertags. This is more easily noticeable in Competition Public Freeplay, where everyone will be human, but a disguised Operative will show his victims Gamertag above his head, even after his victim has respawned. This can lead to SEVERAL (I once saw an entire team disguised as me) duplicates. These GTs will not show through walls, so if an ally's GT shows up just as he enters the room, he is an impostor.

An operative will almost always act a little busted if you stop and ironsight them. They may sprint away, stare at you waiting for a reaction, etc. The sketchier ones will pull their weapons, breaking the illusion.

Failing all of that, simply set up a private 1v1 with a friend, both play as operatives, and take in turns killing eachother and wearing their skin.

This one is easy. Have a private match with another friend in freeplay and both be operatives. The one going for achievement gets killed then the killer takes disguise and then you kill him when you come back. Bee Boop Acheevo unlocked!!!