The description for this Achievement is pretty straight forward become a nemesis 50 times. To become a nemesis you have to kill another player 5 times in a match without them killing you. You can track your progress in the menus under Track Record>Feats of Prowess>Lifetime>Nemesis Awards Earned.
Here is a strategy if you are having trouble.
Make one of your custom load-outs with the following
Body Type - Mighty
Weapon - Jackhammer/Either Shotgun
Mod - Penetrator ammo (Helps with other Mightys wearing armor)
Secondary - Gatekeeper
Mod - Either sight (in-case you get into a gun fight at a distance gives you a ranged accurate weapon)
Gadget - Glider
Support - Armor
Fun Fact 1 - Horse Healthy (adds health)
Fun Fact 2 - Kinda Bulletproof (decreases bullet damage)
Now as this character use the air vents to fly around the map and use dive-bomb by holding the right trigger while flying. You have a good chance of killing your opponent with the dive-bomb but if you don't you have either a huge machine gun or shotgun to finish them off. Knifing your opponent will also work most of the time.
*With this strategy watch out for airspace deniers (for obvious reasons) and players wielding hatchets (one shot kill on anyone no matter perks, armor, or body type) being so big you are an easy target for this.
Another strategy a friend of mine Sawman and I have been using recently is to take turns being a medic for the other. Using the same load-out above but your friend has a motivator and the care-package. On a map like amusement mile go to one of the choke-points, have your friend throw down a care-package and then use the motivator while you and the rest of your team mop up. If you can choose a choke-point next to a heal point already on the map you are practically indestructible.
*sirbradenwalker had a great idea, having a friend use the targeting goggles the entire match. That way you would be able to find the same character over and over and pick on them. sirbradenwalker managed to get 3 nemesis awards in the first match he tried it. Thanks for the suggestion! If anyone else has good strategies please let me know and I will add them to the solution.
*PointySpred gave the load-out below which includes some DLC weapons that didn't exist when i wrote the initial solution.
I found a slightly different load-out that worked a lot better for me, plus I was able to do the weapon challenges with this strategy.
Body type: Burly
Weapon: Falcon blade
Mod: Penetrator ammo
Gadget: Roller skates
Fun Fact 1: Horse healthy
Fun Fact 2: Kinda bulletproof
Rampage: Eagle eye
With my " Burly Ninja" ( I used all my coins to get the ninja and samurai costumes completed). LT is your best friend. You can take a full clip easily while blocking and when they reload you move in for the kill, its too easy. Hilarious to do circles around a health station and watch someone run completely out of ammo then try to run away. When you see everyone at a choke point, immediately take the long way around and flank, use falcon to mop up everyone not watching their six! You don't show on radar. Running around with your blade I'd say about 60% of the time a teammate would start to follow with a motivator, making you unstoppable. Doing this will get the falcon challenges done too. Make each swipe count, don't run in slashing away, keep moving, use the skate ramps for quick escape, learn the maps and choke points. Plus its bad ass when you unlock the gold paint for your blade.