how to train skill

[Sneaksie]

You should use different weapons for close quarters and open areas. Best assault rifle won't help you in close quarters, because enemy with a pistol will aim much quickier. And yes, merc develop a habit for a particular weapon he/she uses.
Fighting alone is too hard IMO, in addition to your stealthy main character you'll need at least machinegunner for support, someone to carry and use heavy stuff like grenade launchers, a good medic... and what would you do in case of daytime engagement? What makes this game incredible is adrenaline level simulation, several enemies would suppress your lone hero easily (and there are fights against several dozens of bad guys, you know).