It's possible to do this in one run, although it does seem to remember how many you killed in a previous run.
Pairing this with the "Revenge" trophy in Yamantau is also a clever boosting tactic.
Doing this nets you 90 human kills. Killing all bandits on Volga nets ypu around a further 35-40+ (depending on how many the game spawns moving around the map to smaller outposts.
Killing all bandits on Caspian nets you a healthy bonus.
The game does NOT include any enemies killed by the AI. So, for example, if Damir does most of the kills in the fuel depot in Caspian, all those kills do not count.