After few tries i got this achievent. It's not random like lot of people think, YOU (and not some of your teammate) need to last-hit the boss. I killed him lots of times but never got the achievement cause i usually play medic and he deals low damage so is hard to last hit the boss in the huge confusion that the battle makes. I got with a lvl 20 commando on Hard with the SCAR, but best perks are the ones that have huge burst damage, like demolitionist or sharpshooter. To be sure, in the final screen after a victory, you need to see in the stats that YOU have killed Hans Volter and the achievement will be yours.