There are a total of 7 Expeditions that you access either from your base or from the Mission log.
The first one gets unlocked fairly soon in the story the next 6 you unlock by finishing rank 1 of the previous one.
Each of the Expeditions can be replayed and each time you replay and finish one the difficulty rank ups by one (up to rank 3). So in order to do all 7 at rank 3 you need to play and finish each one 3 times (so a total of 21 succesful expeditions).
All of them are pretty straight forward -> get the package -> go to extractions -> wait until heli arrives while killing baddies -> get in helicopter
I would advise going for this achievement either late in the game or after finishing the rest since the rank 3 enemies can be pretty rough and swarm you at the extraction . The good weapons , extra life bars and perks will help for the high ranks.
Make sure you have at least 1 close quarters high damage weapon and a medium distance one (assault rifles work). High level bows are pretty effective killing machines if you are good with that. Have all your medkits replenished before starting and make sure you have your Gun for hire with you .
*** Spoiler - click to reveal ***Some of the later unlocked perks (Wrath) make high rank Expeditions way easier. Activate Wrath whenever you are in danger of dieing and start punching enemies to oblivion.
Happy hunting!