Just got this achievement today. If you have beaten the game and are able to help the blue lost souls this is what I was doing. I was starting the game and listening to if he said one of his pals was nearby. That meant there was a blue soul on the first level and nearly every single time there was a yellow/gold souls as well. So I was just running by all enemies till I found either and getting 6 kills and then finding the other and getting 6 kills.
I got the achievement on accepting a gold soul which was unfortunate because I was hoping it would be a blue so I could confirm if they counted.
This has to be the quickest way to get the souls out of the way. 3-5 mins for 1 to 2 souls if blue counts.