So, I have Explosives, Strength, and Firearms all maxed out.
I have driving and agility both at 4 stars, which is enough to get me the achievement for 'mastering' all 5 skills.
I only have 2 bosses left, though the main guy (Wang) almost got taken out early. I was messing around, just experimenting, when I found myself making good progress through his tower. I got face to face with him, and lined up with my Firefly (rapid fire rocket launcher), when I got knocked off the edge of the tower. If not for that, I would have taken him out before I had taken out half of the Shai-Gen bosses...