Alyosha doesn't get wounded.

To unlock this achievement you need to ensure that Alyosha does not get wounded at the end of the Taiga level. The requirements for this are basically the same as for "Forest Child" where you will need to complete the entire level without hurting any human enemies, though you can get spotted by them and this achievement will still unlock, unlike "Forest Child". The best way to approach this is via the same method detailed in "Forest Child", using the quick save feature to your advantage and making small gains in progress before saving, then making more gains, etc., reloading where you get spotted or make a mistake.

You will know you have achieved this during the final confrontation with Olga when you and Alyosha zip line back to the Aurora when the children of the forest don't attack you during the cut scene.

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 "Damir".