Injustice: Gods Among Us

this is my first solution i have written, sorry if its not super great.

after many many many hours of online play with all match types, and using an excel spreadsheet to track wins/loses and total matches played for each game type.
this achievement seems to only include ranked and player matches <1vs1 private counts>
private koth and private survivor did not seem to count.

one other issue is the achievement pop.
this one does not give the satisfying "pop" sound or on screen notice.

this is very frustrating after many many hours of grinding just to have no gratification.

hope this helps those out there, still grinding away.
find a friend, win 100 and loose 100 and both get this and the "over the top!" play 200 matches achievement :)

Me and my friend WarmedxMints who is absolutely awesome discovered that you can easily do this with turbo controllers if you have them.

Easiest if both people have turbo controllers as you can leave it unattended.

Start Multiplayer > Private Match > 1 v 1 > and invite your friend in. Have the person who is winning choose the Joker and the person who is loosing choose Batman. I'm sure others work as well, but this is what we used. The bonus with the Joker is that his (A) attack also is a step forward. This will push your opponent against a wall and enable easy destruction. To make things easier on you, I suggest having your opponent turbo (A) + (Y) as with Batman he just changes stances back and forth leaving himself wide open for attack.

With buttons turbo'd as described above, this setup will rematch game after game. Once you hit 100 wins, stop and change characters around and then let your partner get his wins in. As time goes on you will get a streak bonus that gives progressively larger amounts of XP. Strongly suggest letting 1 person get all their wins in as this will maximize XP gains.

Between load times and the fight itself, this averages around 1 minute per match for a total of 100 minutes (1 hour, 40 minutes) per booster. (Perfect amount of time to watch a half-decent film.)

This is a solution for the ones out there who don't have a boosting Partner or aren't into boosting. In other words, this is for the ones who want to unlock the achievement legitimately.

Your best bet is to use Deathstroke. He can do some damage with his melee and he can deal a good amount of damage from afar with his machine gun and dual wield pistols.

GUN COMBOS
~The most powerful gun combo you can use is the machine gun one : Down+Forward+Y
If your oponent is crouching and your bullets aren't dealing much damage to him, then meter burn this combo (Press RT at the end of the combo)
~The other gun combo you can use is the dual wielded pistols. It's not as powerful as the previous one, but it's way faster. Use this combo when your oponent is getting too close from you or when he's preparing a combo. Sometimes, you'll face oponents using Deathstroke and using the exact same strategy as you (Spamming). This combo will prevent them from using their powerful machine gun special attack. To use it: Down+Forward+X
~If your oponent is jumping, use: Down+Back+Y.
~If your oponent is crouched and won't move, use: Down+Back+X

Now, you know the combos.

First, you're going to want to win 10 Ranked matches to earn and progress towards your 100 wins. For this achievement, spam your gun combos until your oponent is K.O.

Once you've unlocked this, go in Player Matches and then choose Player 1v1. Accept an easy oponent (More losses than wins or a small TrueSkill level.) In Player Matches, your strategy will be different because the game won't count if your oponent leaves the game before it's over. So, what you're going to do is mostly use melee combos against weak/noob oponents so that they don't think you're cheating by spamming your gun. This way they shouldn't leave the match before the end. In fact, I,ve noticed that most bad players even want a rematch at the end of the game, giving you another easy win.

These were the melee combos I found useful:
~ If your oponent is gaining the upper hand in a melee battle, use: Down+A. Your oponent will fall on the ground and your oponent won't gain confidence. Also, using this combo is very helpful if you want to move away and perhaps use your gun. It's okay to use your gun, as long you're not spamming it.
~Back+A does a lot of damage and it allows you to either juggle your oponent or make a level transition.
~Forward+A Deals a lot of damage and hard to counter.

If the fight is getting tough or you realise your oponent is better than you, dash away and spam your gun combos.

The characters you want to avoid are WonderWoman, Batgirl and Scorpion, If your oponent chooses one of these, simply put pressure on him by not chosing your character, hoping he'll choose another one.

Use your meter to your advantage! Remember you can Clash to earn health, use your special attack to damage your enemy and meter burn your special moves. Try getting the first hit in the fight to fill one bar of your meter to use at your advantage.

Win streaks: You earn EXP boosters when you're on a win streak.
At 5 wins: 5minutes 2x EXP
At 10 wins: 10 minutes 2x EXP
At 15 wins: 15 minutes 3x EXP
At 20 wins: x minutes 3x EXP
At 25 wins: x minutes 4x EXP
At 30 wins: x minutes 4x EXP
At 35 wins: 60 minutes 5x EXP
At 40 wins: 90 minutes 5x EXP
At 45 wins: 30 minutes 10x EXP !
At 50 wins: 60 minutes 10x EXP !!!

These EXP boosts can be used in versus to make progress towards level 100. Before your EXP boost expires, start a versus match. Now, your game can last hours, days as long as you started the game before it expires, you'll benefit of your EXP boost! Imagine how beneficial it is when you have a 10x EXP! Use this method: Solution for The Hero We Deserve In Injustice: Gods Among Us
I was able to have a 70 win streak (Still ungoing), so you can too!

My final stats were: 215 games played - 170 wins - 70 win streak (ungoing)
So it really is this easy!

Lastly, be prepared, you're going to receive a lot of hate messages! I got one every 3 games in average.

If you want to get a feel of the strategy, look at the first minute of this video. Note that the video is not mine and I don't take any credit for it.