Batman: Return to Arkham - Arkham City

It is recommended to do this with any of the combat challenges, since there are more enemies available.
Personally, I did this one on the Iceberg lounge challenge. What's perfect with this map is that enemies start spawning successively. It is pretty easy to get the 50x combo there if you pay attention to your environments. At the beginning you will be facing only normal enemies without gear or knives. Still, if you are having troubles getting it, you can activate detective mode to make it more obvious, if enemies are of the regular type or not.

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Hope this helps you guys!
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I got this achievement on Hard mode, outside of the courthouse, during the time when you are looking for signal jammers. A fair amount of thugs will be there picking on a random person. What you need to do is obtain a combo of 50x, meaning consecutive strikes without being damaged or losing your streak. The Critical Strikes upgrade is, well, critical for this to make it easier. This is because once you build a streak each hit will count as 2 hits in your combo. Strike a foe or two, and either keep countering, or, the easier and more guaranteed way is to keep evading, by double tapping A and a direction after each strike that you do. Strike, evade, strike, evade, rinse, repeat, and do this until you reach 50x and you will obtain this achievement.

I got this one pretty easily from the Mad Hatter side mission. This side mission becomes available shortly after the Mr. Freeze boss fight and has an achievement attached to it called “Stop The Clock”. Alfred will call you and send supplies from the Batwing in the form of a drop point. The drop point is in NW Arkham. When you reach the drop point it becomes clear that it’s a trap. The Mad Hatter has poisoned Batman and he begins hallucinating. You’ll get to a point where you’re standing on a giant clock face and you have to fight large groups of thugs with rabbit masks on. This is where you get the x50 combo. Tons of thugs and if you mess up you can easily retry and start again at the beginning of the fight. I wouldn’t try to do anything fancy, just attack and counter when necessary and you should hit a x50 combo easily. By the end of the mission you’ll get 2 achievements out of this (Stop The Clock and 50x Combo). Not bad.

I got this right in the beginning of the game after you break out of the cuffs when you first meet Penguin. As soon as I got up I knocked out the Penguin finished off the rest of his goons and before I left the area I started beating the sh*t out Penguin while he was on the ground until I had a 50x combo. This method only took me a few minutes but it is kinda difficult cause you will slide off of him sometimes if you're not careful.