The achievement requests that you add 4 Legendary Lieutenants to your ship crew and have them equipped.
Note:
- You will need to upgrade your ship's hull until you have 4 lieutenants available, otherwise you can't equip them all and complete this
There are 2 ways of doing this:
1. Scan using Ikarus for a legendary enemy, usually they are Captains or Polemarchs or even Mercenaries like you (they have the golden background behind their picture). Lower their health to basically one shot then hide.
Approach them again from behind and use a non lethal takedown (LS on X1 controller as i didn't play Mouse + keyboard), after they get knocked approach their body and you will receive the option to recruit them, go to Ship - select one of the slots and equip.
Repeat x3 times and done.
2. Finish the main story and you will receive some of them - i got 4 - (i will avoid to explain which one you receive in order to avoid spoiling) - this is the way i have completed it.