Sculpting and ZBrush pass
When all the pieces were modeled I export them to ZBrush as separate subtools and began sculpting step-by-step: modifying shapes, adding more details – wrinkles, scars, hair, veins, folds etc.
For the body parts I used layers for the scars, veins and skin texture. At the final stage my .ZTL was about 60 millions (without weapons) (Fig.07 – 10).

Fig. 07

Fig. 08

Fig. 09

Fig. 10
Low poly model
After the high poly model was modeled and sculpted, I exported the lowest level of all of the subtools to OBJ one by one.
When all the pieces were in Maya, I grouped them into blocks depending on my low poly needs and started retopology using Maya's standard tools for polygonal modeling
When low poly was done, I continued with UV mapping inside Maya (Fig.11 – 12).

Fig. 11

Fig. 12
When the low poly model was done, I did some preparations before exporting pieces for baking. Parts that could potentially cause bugs when baking Normal , Cavity and Ambient Occlusion maps were separated from each other.
So every low poly piece was exported as a separate object to make the process of baking easier and much more predictable. By the way, before exporting meshes for baking I did triangulation to avoid Normal "shifts" in the future. Doing this helps the geo look much cleaner with and also don't forget about hard edges at surface critical angles! (Fig.13).

Fig. 13