Killer Instinct

For this achievement, you have to go in "Multiplayer", "Ranked Leagues", and play ranked matches. You need to win against 10 players that are in the "killer" rank. It can be the same player over and over again, as far as I noticed. I got it after winning against someone I had already defeated. However, unlike the "win 20 ranked matches with XXXX", what counts as win is winning points in the match. So, if you have a three round match, it only counts if you win two rounds, and it only counts as one win. In a similar way, if you get into "observation" for winning too many disconnected matches, you DON'T get wins anymore. This is also different from the "20 ranked matches" achievement.

Another way of writing the requirements for this achievement would be "Get points in ranked matches against Killer opponents 10 times".

Best way to get this would be farming with a group of players, since most "Killer" players have a lot of time in game, and winning against them requires a lot of training. You can block players in Xbox and you won't match them anymore. This can help meeting in boost sessions.
FOR THIS NEXT TRICK, USE AT YOUR OWN RISK FOR BEING PUNISHED AND RUINING OTHER PEOPLE'S EXPERIENCE! If you can't boost, you can try to get a disconnection. If you make your internet slow, or plug an ethernet cable while playing, you may be disconnect and receive a victory. HOWEVER, you will get into observation after some wins, and at this point, any win you get won't count for this achievement.