Batman: Arkham Knight

Potential spoilers:

This is a MAIN storyline achievement, that is achieved over 90% through the main campaign. Towards the end of the campaign you will meet with Oracle in the GCPD and all of a sudden the building will begin to rumble, cue the Scarecow broadcast. The building is under attack and you must proceed to the parking lot, once their militia vehicles will arrive and men will hop out to fight you (also the power goes out). There are a few medics here so they take priority, also make sure to make use of your BATMOBILE takedown. Once they have been dealt with, hop into your batmobile, go into guard mode and use the winch to pull down the extractor fan looking thing. After a series of vents and grappling (with Joker commentary to boot) you will arrive outside to find 60 militia drones in wait. Oracle will update your objective and you will be tasked with restoring the generator. You will arrive at the generator to find it destroyed and will then be told the back-up generator is in the parking lot (which you have to access by the normal front door that you have been using throughout the rest of the game).

Once inside you will again find a group of militia to take down in the dark. When they are all dead power-up the generators on the wall using the remote electrical charge and summon your Batmobile. Oracle advises you she can help with the drones and you head out the same entrance you came in and take them on.

Once these have been destroyed, Scarecrow will land a Helicopter on top of the building and send out a wave of enemies. Head to the top of this building and take them out (first wave is pretty standard). This will then be followed by two more waves and once the third wave is down the achievement will pop. Warning: This is probably the hardest non-batmobile combat in the game so play defensively and watch those counters!

Tip: When fighting large groups of enemies I find it easiest to take down the medics first, if you see a medic trying to revive someone, he is your priority. After the medics I tend to favour the brutes just because you can get a beatdown and then counter/evade if need be and go straight back into the beatdown (and this racks your combo up for special takedowns). Next prioritise the enemies with annoying gadgets/shields/swords and then finally the normal guys.