Load up the Silent Knight predator challenge. For this one you need to use Batman and/or Robin, do not get hit, and only use knockout smashes. A knockout smash is when you start a silent takedown from behind with
and then press
.
The key to making this one easy is taking out the medics first. Once the medics are taken care of, you can take your time mopping up the remaining guys. There are 6 total enemies consisting of 3 medics and 3 regular thugs. The regular thugs have guns, so if spotted, be swift at getting away. If you see your health bar appear in the upper left, you've been hit and should restart.
Also, if you're having trouble getting all 3 medics down in quick succession, be aware that once a medic is revived, they lose the ability to revive anyone, so you won't have to prioritize a downed medic that has been revived.
The easiest way I found to down the 3 medics quickly was to grapnel to the vantage point in front of you, and then across the map to the office area, and jump down on the roof. From there, you can knockout smash the medic that walks out; quickly corner cover to the left of the doorway that leads out to the catwalk, and another medic will come to check on him. Knockout smash him, and quickly grapnel to the vantage point directly across from where you first started. This will position you to see the 3rd medic climb the ladder and head towards the office area; drop behind him and knockout smash him, quickly grapnell-ing from between 2 - 3 vantage points to get away. At this point you can mop up the remaining 3 how you see fit; however I found it easiest to drop down into the grates on the lowest level, and take out the patrolling thug there. Another thug will come and check on him, and you can easily take out the 5th guy. At the point the 6th guy should be terrified, and will have somewhat erratic movement and turn around often. Perch on a vantage point and when he gets close, drop behind and do the final knockout smash.
For those that prefer a video, please see below. All credit to the original poster: