How can a Public Relations Professional balance advocacy and objectivity?

Leen dear, you probably mean propaganda and not advocacy as advocacy is another important area which requires special skills. The way a good PRO can create a balance between propaganda and objectivity is to ensure the objectives of his company are met, his company gets what it deserves and also the company reputation doesn't suffer because of subjective thinking and false information or propaganda by her//him or others.

My dear 

there is one efficient way i have had as PR specialist by use every available tool that i have in my company / organization to achieve their goals objectively including creating strong mental image of organization , strong links with mass media, building trust between CEO,s and employers.

You can consider this as kind of strong balance between advocacy and objectivity