“dans l’âme” is a French expression meaning that the quality in question is part of the deepest nature of the subject.

“Mozart était musicien dans l’âme” means that music was deeply part of Mozart’s nature.

“Le maréchal Joukov était un stratège dans l’âme” could be translated by

Marshal Joukov was a born-strategist. Of course the invoked soul has disappeared in the translation, but I would say that born-strategist is the standard way to say something like this in English.