Start up a beast game on casual on any map(preferably Thrashball).
Play as only a wretch and kill everyone for the first 3-5 waves.
Fail when it gets too hard and restart from beginning.
Rinse and repeat until you get 200 kills.
*For multiple people*:
If you have a friend, this will make it easier.
Go in at them at the same time.
Have you or your friend stun scream then attack.
Take turns doin so and it should be easy kills.

Make a private versus match with these settings:
King of the Hill
RSL:240
Rounds to win: However many desired
# of Bots: Fill
Bot Difficulty: Casual
Make sure you have those, or any variation of those.
Basically all this mutator is is getting silver medal with all 5 starting weapons(Master at Arms Medal).
Just start the game with the above settings and get kills with those weapons until you unlock the Master at Arms Medal.

Make sure you have another controller.
Sign that controller in and set the party status to Local.
Make sure that you can buy turrets with your main controller.
Start a game of Horde with these settings:
Map: Thrashball
Difficulty: Insane
Start the game and give all of the second controller's money to the main controller.
Buy a command post and then a turret.
Have the second controller get on the turret and spray.
With your main controller, repair the turret as it's spraying.
When you get the Combat Engineer ribbon, you can keep doing it until the 30 sec ends to help get the Field Engineer medal boosted.
Then just simply let the locust kill you and start from current wave again.

Just basically set up a private versus match with these settings:
Gametype: Warzone
Rounds to win: 1
# of Bots: Fill
Bot Difficulty: Casual
Just basically be the MVP every time and you will eventually get the Onyx medal+Superstar Cole
This method is also the way to get Match Winner Onyx.

This one is very easy, but takes time.
Set your character as a female character.
Play versus a lot.

This is probably the easiest out of all of them.
The method is exactly the same as the Infinite Ammo mutator method.
This method can be done online as well though.
By the time I got the Infinite Ammo mutator, my Field Engineer was already Onyx 6.

This is very easy but takes time.
Start campaign from the beginning(not Prologue.) When you go to get Jace, you'll notice a thrashball machine. You can press X on it and that = a manipulation.
Continuously press X to get the medal.

For Match Winner, rather than playing, you try Execution, 1 minute, 1 Round, 2 bots, casual.
You will have one more bot than the other team.
The bots will never kill each-other and the round will end, and because you have more people on your team who survived, you win the match.
Also: When you start the game, and the countdown begins, disconnect your battery pack.
The game will never pause.