In order to get this one you are going to have to play on hardcore. You'll have to score 50 kills in roll while not letting the damage streak bar fall (bottom right of the screen) and be as quick about it as you can. Sounds hard but as long as you are putting shots on a target it won't decrease as quick or run out. Its best to try this on levels where enemies and/or groups are in a closer proximity to one another.
Okay, finally got this. I think the best level for it and the one I got it on is level 3. When taking out the SAM sites (shortly after the secondary objective becomes active). I got a 60x streak, so you more than easily can get it there. Diz really seems to be the key to getting this with her EMP and easier controls. Went with the Hardened Hull, Stasis Shock-Elite, Efficiency-Elite, and the Turbo Boost (a must if you want this achievement).
Here's some great advice by heatDrive88 on how to get this achievement:
*Check below for the full post, don't have enough room for the entire thing*
The easiest place to get a 50x damage streak is on Mission 2, when you have to take on the quest of taking down the mortars.
1) Before you enter the first area of enemies at the left-most mortar, make sure you take a look at the map and mentally note the quickest and most efficient path to reach one sub-area to the next. As you progress halfway through while clearing to the right, there's a jump you can take off a cliff which will help get you to the next sub-area faster.
2) Most obviously, go in with your machine gun at the max level of 3, and bring either rockets or the railgun for your secondary weapon. Homing rockets could help so you don't have to try so hard at aiming, but it's not necessary. Don't use a flamer as it will take too long to kill enemies, and the shorter distance is not helpful.
3) Take the Nitro-Boost ability with you at the beginning before the mission starts. These short boosts will help you get from sub-area to sub-area much faster. Just be careful with it and try to only use it on a straightaway, as one wrong boost at a turn or a twisting jump will send you flying, most likely not in the direction you want to go or into the trees which will possibly end up with you being flipped over. If this happens, you're going to get screwed over and the damage streak timer will run out, and you'll likely need to start the mission all over again.
4) Every entrance/exit from each sub-area to another where will have 1 or 2 bunker towers at them, which will only fire weak gunfire bursts at you. As you exit from one sub-area to another, try to shoot and destroy the bunker tower as you are driving away and leaving from the sub-area to the next. This will keep your damage streak timer as high as possible to give you more time to travel from one sub-area to the next. Be sure to also prepare for the inevitable bunker tower as you approach each new sub-area, and begin firing on it immediately when it enters your view to keep the damage streak going.
5) Don't waste time picking up health unless you absolutely need it. The last thing you need to do is let your damage streak timer run off while you attempt to pick up items or shoot at things that won't keep your damage streak going. If you're in the red on health, then of course, try to pick it up as quickly as possible, but just don't take too long as time is definitely a factor.
6) Lastly, don't be afraid to use your railgun or rockets on the enemy, or your special ability. Some people get gun-shy with them, but if you chose the loadout that I recommended, they will regenerate fast enough that you can use your rockets or railgun on every missile truck enemy you run into, and possibly more. Getting hit by an enemy rocket can devastate you health, so you need to dispose of those missile truck enemies quickly and efficiently. If you die partway through your damage streak attempt, you will most definitely have to start the mission over again for another attempt.