Gears 5

This can be done in a PRIVATE MATCH! We will be getting all the Free For All achievements in the Operation 3 DLC in one match.

If you just want this achievement then load up a private FFA match with bot difficulty set to beginner and don't kill anything until the bot in first place has 10+ kills then go to work killing everyone you see until you win.
If you want all the FFA achievements in one go then follow the guide below.

Start the game
Select MULTIPLAYER
Select VERSUS
Select CUSTOM ONLINE
Select HOST PRIVATE at the top of the screen
Select GAME SETUP
Select GAME MODE
Select FREE FOR ALL - 14 PLAYER then go back to the previous menu
Select FREE FOR ALL - 14 PLAYER OPTIONS
Turn FILL WITH BOTS to ON and BOT DIFFICULTY to BEGINNER
You can also adjust your ROUND SCORE and/or ROUND TIME LIMIT here as well if you need to make it easier.
Now go to WEAPON SUBSTITUTIONS and change all weapons to gnasher except for the breaker mace and the torque bow. Change one of those to lancer and the other one to gnasher.
Now go back to lobby and pick CORE for your map and start the game

You're going to want one kill with each weapon you load out with. Lancer, gnasher and snub. Since those weapons are all on the map as the only pickup weapons you don't have to pick up anything.

The first thing we'll be doing is nothing. Just watch the scoreboard until the bot in first gets to 10 or 11 kills then start killing everyone you see. You'll end up unlocking Line Them Up for killing 10 different enemies in one match, Come From Behind for winning a match where you were behind 10 or more kills and Victory Seized for winning a free for all match.

Just to add to the guides you can do this with two controllers and no bots