Duke survives.
To unlock this achievement you need to ensure that Duke survives during the final encounter of the Volga level. Here is what needs to be done:

During the opening mission to infiltrate the church, DO NOT kill any of the cultists. You are free to knock them out via stealth, but you cannot kill them.

During the mission to commandeer the merchants barge, you again cannot kill any of the enemies as these are also cultists and killing them will give you negative karma.

Finally, the mission that ends the level where you infiltrate the bridge to lower it and allow the Aurora passage, again do not kill anyone throughout the mission.

Don't kill any surrendering enemies throughout the entire level, though you shouldn't be fighting any if you want to maintain high karma.

Use stealth to knock enemies out or avoid them. If playing on easy you can just sprint to the objective to force the next section/complete the mission as you will not take enough damage to kill you in almost all circumstances.

You will know you have achieved this during the final confrontation with the preacher at the top of the bridge when he tells his men to lower their weapons and allow you to pass unharmed.

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 "Damir" and "Alyosha".