Damir stays with crew.

To unlock this achievement you need to ensure that Damir remains with the crew at the end of the Caspian level. A list of what needs to be done to ensure this;

You cannot kill any slaves throughout the entire level, this includes any that are attacking you such as the slaves that are attacking the light house when you arrive. You can knock them out if you wish, but cannot kill any of them.

You cannot kill any slaves during the mission with Damir to secure the water tank and later on the fuel tank

You need to rescue the slaves in the mission with Damir to secure the water tank, after riding the bucket lift up, see "Roller Coaster" for a location, turn behind you and follow the edge of the cliff, around the lorry and on your left will be a cage with a guard dog asleep in front. Kill the dog, then break the padlock on the cage and release the slaves.

You need to rescue more slaves during a side mission given to you by Anna. The side mission is to infiltrate a container ship to the North of the map and you will be given the side mission after exiting the Archives and meeting Anna in her lookout post. Make your way through the ship killing everyone, the enemies here aren't slaves so can be killed, then at the top of the area there is a switch to open all of the cages, just in front of a zip line that will take you back towards the beginning of the ship.

Same as always, don't kill any surrendering enemies and use stealth wherever possible to knock enemies out or avoid them.

The enemies you kill at the very end of the level when you are confronting the Baron are perfectly fine to be killed and will not void the achievement.

You will know you have achieved this during the final confrontation with the Baron at the top of the oil rig when Giul leaves and Damir makes his escape with you.

This can be done via level select, however for it to count towards "Full Strength" it needs to be done during the same full campaign as "Duke" and "Alyosha".