You can do this in phenix.maps (which runs Phaser for you internally) - just specify "LLG" as the map_type, and you can name the columns whatever you want (FLLG and PHLLG are good choices, but the GUI will fill something in for you automatically).  You should also specify the wavelength, of course, otherwise it's going to look pretty similar to the simple anomalous difference map since no anomalous scattering is modeled.  The f'' won't actually be refined, but it's a pretty good first approximation, and the result definitely more sensitive towards missing weak scatterers.

You can also get the LLG map from phenix.refine (so why use anything else?), but that requires performing anomalous group refinement (there is a wavelength parameter but it is not widely used at the moment).  My tests indicate that Phaser's anomalous refinement is more accurate, however.

This is probably yet another item for the FAQ list.

-Nat