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 just load up a private free for all game with bots set to beginner difficulty and kill everything you see. The achievement will pop once you kill your 10th different bot.
If you want all the achievements for FFA in one go then follow the solution 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.

This can be achieved in PRIVATE match!

Versus
Custom
Online
Host public
Game mode - free for all 14 player
Bots - on & difficulty beginner
Round score 100/50 & time limit 60 minutes
Weapons - tri-shot ALL OF THEM
Lobby browser - not visible
Map - core

Took me 3 minutes.
HAVE FUN