UPDATE: Thanks to AceMcCloud and rafa1000house for news about TU
TU is roll on today, 9am PST on Tuesday 23rd February, and social kills before Title Update DON'T count for "seriously..."
https://gearsofwar.com/en-gb/community/ultimate-edition-feb-...-----------------------------------------------------------------------------------------------------------------
EDIT: Forgot to put the link about Gears TU:
EDIT 1: Thanks to rafa1000house for the exact date when The Coalition changed competitive modes
https://gearsofwar.com/en-us/community/news/ultimate-edition...The Coalition changed competitive game modes on October 30th, 2015 to only Blitz and Execution, so you can't play KotH like the others solutions says.
Well, some people (like me) are unable to find matches like others (Because the country we live/Internet connection/Random/Time we connect/Etc). So playing Blitz it's like 5 times in a week, and Execution is more slower for this achievement (And I can't find any match, anyway).
But fear not!! The Coalition posted about an important update, and social kills will be count for Seriously... :)
The problem is that nobody knows if all the kills made in social before the TU will count, but I'll update this guide when I know ;)