Alpha Protocol

Save manually once you're fighting him. In the convo following his defeat, choose to spare him, and this acheivement is yours. Reload your save, defeat him again, choose to execute him, and you'll get the "One Less Gangster" acheivement.

Boss is much easier to beat if you :
- did Taipei first : buy intel from Steven Heck that will mess with his drugs. If you're stuck at the boss fight without having done Taipei, load last safehouse checkpoint, and in the mission select screen, press RT then LT, and select Taipei safehouse. Try Moscow again after befriending Steven Heck and buying his intel.
- chose the data over saving Albatross/Sie (unless you're playing veteran, in which case you can do both), since 100% intel on Brayko gives you a +5% damage perk on him, while saving Albatros/Sie gives you a 20% endurance recharge rate perk.
- use a flashbang (equip a few of them before starting the mission) when he takes his wiff (or when he's running after you) and take that opportunity (he'll be stunned) to shoot him so as to drain his resistance away (the blue bar above his health bar), then headshot him with a maxed out crack shot (pistol skill). Pistol is the best anti-boss weapon. So be sure to level your pistol skills as much as possible.

"A Price on Mercy" is awarded for sparing the life of Konstantin Brayko during the second to last Moscow mission. You must spare Konstantin Brayko if you are pursuing the "Ask Questions First, Shoot Later" (https://www.trueachievements.com/a45189/ask-questions-f....htm) achievement.