Use the Bodyguard Ability against your pursuer as The Privateer in any multiplayer mode.
The Privateer is the male pirate character with the (default) 3-point hat. On the selection screen he is the 5th down on the left side.
This can be earned in ANY game mode.
You will need to be at least level 15 before you can earn this achievement, as you wont be able to purchase the "Bodyguard" ability before then. If you're not yet level 15 then I recommend starting with The Vulture achievement:
I don't know how familiar you are with the multiplayer modes so I'm just going going to assume you know very little and cover all the important information, feel free to skip any part of this that you don't need.Setting up your abilities
In the Multiplayer main menu you'll want to push LB on your controller then select "Abilities Sets". After that you'll want to buy (with Abstergo Points) or select any of the non-default sets.
Select "Ability 1" or "Ability 2" depending on if you want to assign the "BodyGuard" ability to the LB or RB button respectively. Once you selected which ability you want to assign it to you can buy the BodyGuard ability for 50 Abstergo points.
If you have Abstergo points to spare I recommend adding at very least the faster cool-down perk as this will help you earn this as well as the other DLC achievements faster.
Make sure you select this ability set when entering a multiplayer match, if you forget you'll have the option to change it whenever you die by using the D-Pad and A-button.Setting up the game
This achievement can be earned in any game mode, public or private, however I recommend selecting Custom Match and searching for a Free-For-All Deathmatch.Using the Ability
Basically when you push the button to use this ability the game will morph a near by civilian and they will attempt to stun your pursuer, giving you time to escape. The bodyguard doesn't always stun your pursuer, they might be killed before they can. In order for it to count toward the achievement your pursuer must be stunned.
Once you use it you will need to wait for the ability to cool-down before you can use it again.Strategy
This is a fairly straight forward achievement though I found that the quickest strategy is to sit on a bench next to a civilian or blend with a crowd standing around and listen carefully for the "whispers" noise that means a pursuer is near by. If you can figure out who your pursuer is use the left trigger to lock on to them, it's not necessary but it helps greatly. Then use your body guard ability and hope that it manages to stun your pursuer. If your body guard didn't stun your purser wait for the cool-down and repeat until the achievement pops. It should pop the instant you've used it successfully.
I welcome any other tips or strategies for earning this achievement. Thanks for reading!