I find iZotope, and the "Dialogue Isolate" module have worked well for me in situations I thought were impossible.
We do an educational event each year in a college gym of all places, where the gym is filled with several hundred people divided into 10 person tables with medical students group-interviewing patients.
We use Tascam or Zoom recorders at each table to capture audio of the interactions our client wants for training purposes.
Starlight Video Productions
Sugar Loaf NY