Batman: Arkham Knight

My first solution, but here it goes.

As the achievement states, you need to take out 50 enemies during this challenge. This is a timed but continual predator challenge. When you get to a low threshold of enemies on the map, a new wave will spawn in.

**EDIT 5/6/2016**
As RSDAY Commented, This achievement will pop during the level, as soon as you hit 50 enemies, not after you run out of time or die.

Now for some helpful tips.
-I found sabotaging the medics with the disruptor was always a good strategy. Not only does this take one enemy out and adds it to your count, but it also stops another enemy from being revived.
-I had some trouble with the machine gun militia henchmen. I would avoid them, but if they are causing trouble, you can use the voice synthesizer to lure them to a gun box sabotaged with a disruptor, or near an electrical generator and blow it up with your remote hacking tool.
-Eventually, they go crazy with laying mines on the ground. Not a bad thing for me as my play style was hop from vantage point to vantage point. They can mine a vantage point, but the disruptor again can fix this.
-Inverted takedowns are an easy source in this map as well, as they will shoot the guy down from the vantage point for you, freeing the vantage point up for another inverted takedown.
-This map has a count down. It starts with 5 minutes and you will get +25 seconds for each silent take down.
-Fear takedowns can be your best friend. A fully upgraded fear takedown can give you a large amount of time as you can take down up to 5 in a row. This was a rarer occurrence, as I could usually only get 3 or so at a time.
-Optical camouflage henchmen can give you a surprise sometimes, but just keep an eye out for them.
**EDIT 1/10/2017**
-lokisapocalypse comments that at around 40 kills, you can get a good amount of kills from using the disruptor on the mines as enemies walk over them, and i remember there should be quiet a few of them around by the point you get to 40+ kills.

Took me about 10-13 tries to get into a rhythm on this, but it can be done if you can use gadgets effectively and have just a bit of luck with the spawns and how intense the AI is acting.

**EDIT 8/17/2016**
Freakzilla8250 coments that they had it pop not in one sitting, so this might work for some

This achievement is currently Glitched as of 2/6/2020 I have just unlocked this by starting the Endless Knight AR Challenge and getting 1 or 2 takedowns then let yourself be killed and go to the main menu and continue story from here proceed to knock out enemies until the achievement pops.

Video Credit: Git Gud

https://m.youtube.com/watch?v=MMm4UIIkgIs

I got this first go thinking it would be a lot tougher than it was: There's honestly no need to over-complicate things, I didn't even need to use Bat-vision (I hadn't played in so long I couldn't remember what button it was)!

• Stick to the vantage points, no need to risk ground-level shenanigans: The floor gets mine-heavy after a while and by sticking up high I got to 52 takedowns without being seen
• Your 3 best friends: Inverted Takedown, Smoke Pellets & Disruptor Shots
• Smoke Pellets and Disruptor shots will regenerate, unlike in the other predator challenges!
• If you do see a golden opportunity (isolated bad guy with no mines around) for a silent takedown, take it
• Don't worry when they start putting mines on vantage points, they won't cover them all and you can quickly land on one and grapple off again if you're cornered

My technique:

1. Grapple up to the vantage points & perform an Inverted Take Down on some jerk, and grapple away
2. Once a crowd arrives to cut him down, toss a smoke grenade in the middle of them
3. Glide in and perform 2/3 silent takedowns
4. Grapple out before the smoke clears
5. Crack out a multi-fear takedown at this stage to top up your time and get your timer count up
6. At this stage they should start laying mines, which is going to make things really easy: They should be bunching up like sheep and you can take out 3 or 4 of them with a single shot from the Disruptor onto a mine
7. Repeat this 3 more times to use all your disruptor shots up, but only when multiple enemies are next to a mine
8. Whilst you wait for that Disruptor Recharge, repeat the Smoke Pellet technique or perform a Multi-Fear Takedown
9. If both smoke and disruptor are recharging, look for an opportunity for an inverted takedown/silent takedown

This technique should get you over the 50 mark.

The previous author (buzzo9neo) had some great general tips, and I relied heavily on his solution to get started with this AR challenge.

Here's a few more tips and a 50 takedown walkthrough, based upon the number of takedowns and corresponding number of henchmen spawned:

(1-6 Takedowns): At this point there will be basic thugs spawned, and most of them can be handled with silent takedowns (very important in later phases as it gives you "Fear" for large fear takedowns). At 5-6 takedowns, a detective mode jammer thug or detective mode tracker thug may spawn. To quickly locate the jammer thug, turn on detective mode and look for the red triangle. I frequently rely on detective mode in predator challenges, especially when setting up fear takedowns, so my priority is always to find jammer thugs and take them out. You can also sabotage their jammer devices with the disruptor gun tool.

(7-21 Takedowns): Now there will be a mini-gunner (big guy), 2 medics, camouflage thugs, and regular thugs, as well as a tracker or jammer thug. Silent takedowns are still pretty easily performed here to gain "Fear." Thugs going up or down stairs, even with a "buddy", are less likely to turn around and spot you. Use the map's different levels to create line of sight problems for the other roaming thugs. You can also use your disruptor in a variety of ways, including: sabotaging medics before they revive thugs or sabotaging weapon crates and using voice synthesizer tool to send the mini-gunner to inspect the crate (Note: You can only do this once before the Arkham Knight tips them off that it's a trap).

(22-50+ Takedowns): This is where I found that the AI became more difficult, but maybe it was the increased number of thugs, who seem to roam in big packs. Ten or more of them in close proximity makes silent takedowns tricky, but not impossible. The maximum number of 13 thugs are roaming after 30+ takedowns. If you get spotted, use smoke grenade or grapple + boost above the vantage points and glide back quickly to a takedown position. Since this is a race against time, try to spend as little time as possible up in the rafters after an escape from alerted thugs. The regular henchmen start dropping crazy amounts of proximity mines towards the end of the challenge, and this makes both floor and silent takedowns more challenging. Luckily, you can use your disruptor gun to sabotage the mines, which KO's any nearby henchmen (Note: tool upgrade may be required, but most players attempting these AR challenges have at least most of the tool belt upgrades). Your disruptor refreshes its charges every minute or so, but it's best that you use the charges sparingly, so that you have them when a medic or mine needs to be sabotaged. Inverted takedowns are great fillers later in this challenge, and they are great for launching into big fear takedowns in open areas after the thugs gather under vantage points. (Note: proximity mines will harm you and interrupt your fear takedowns, so always disruptor sabotage them nearby before staging a big fear takedown)

This is a very fun, but sometimes frustrating AR challenge. Hopefully the tips and walkthrough above make it an easy pop for some of you! Thanks!