A Sweet Little Ha-Hacienda
The intro sequence simply involves escorting Joker down to the Holding Cells. Just follow him and the guards with and, when prompted, try out the zoom function with . It won't take too long before the Joker manages to get free of his guards and set free a few of his thugs down in holding.
Beat the thugs into submission () and counter incoming attacks with . Counter whenever you see an enemy about to hit you—you'll see blue lightning bolts above the attacking enemy's head when it's time to counter (NOTE: On hard difficulty these lightning bolts are removed making counter timing a bit more difficult). If an enemy is lying stunned on the ground you can finish him off with a ground takedown (+) but be warned: this leaves you wide open and other enemies will take advantage of this to attack you. +
With Joker's thugs handily dispatched, turn back around toward Commissioner Gordon and look along the bottom left of the wall for a ventilation grate. Rip the grate off with and head inside to collect your first Riddler Trophy. Once collected, follow Joker out through the path he just unlocked to the Processing Corridor. Along the way, be sure to destroy the chattering Joker Teeth with your batarangs. There are three in an office to your left as you leave holding and another two in the Processing Corridor.
A pair of thugs will harass you in the corridor but shouldn't pose much of a threat. Knock 'em out and head down to talk to guard Eddie Burlow.
Shocking, Isn't It?
It seems Zsasz has broken loose and is holding one of the guards hostage. Follow Eddie's directions to Zach Franklin who will fill you in. If Zsasz catches glimpse of you, he kills the hostage—too bad for Zsasz, he'll never see you coming. Climb the nearby stairs to the observation area and activate detective mode with to highlight interactive objects like gargoyles. Pick a nearby gargoyle and hit to grapple up to it. Swing from one gargoyle to the next until you're poised to strike from behind Zsasz, execute a glide kick with , and then put him out of commission with a ground takedown.
Harley will try to lock you in the room but, luckily, thinking is not really her thing. You can use detective mode here to make a couple vent grates stand out: one near where you entered the room hides a Riddler Trophy while another by the wall covered in flat-panel TVs is your ticket out of the Patient Pacification Chamber (and also conceals yet another Riddler Trophy).
A Touch Gassy
After you've made it through the ventilation system, drop down and dispatch three more Joker Teeth before making your way to Decontamination which, of course, is incredibly contaminated at the moment. The room is flooded with Joker toxin and it's all up to you to rescue the sorry saps trapped within. Look for an opening in the ceiling and grapple up to gain access to the room via another ventilation duct.
Stay high above the gas as you move from platform to platform, rescuing the guards along the way. There is also a Joker henchman hanging on for dear life here. Rescue him as well for an achievement (NOTE: This achievement can be missed. If you don't rescue the thug here you'll need to start another playthrough to get this achievement). After you've grabbed the baddie, take a look downstairs to locate the fan control box (detective mode will make it stand out) and smack it with a batarang to clear the toxic gas from the room.
Before you leave, there are three more Joker Teeth to cut down to size and a Riddler Trophy hidden beneath maintenance hatches on the floor.
Follow Joker's convenient green arrows to the Transfer Loop where there are two more Joker Teeth and a pair of generic thugs to give the ol' fisticuffs treatment to. Keep down the hall to finally catch up with Joker. He's got a present for you: a horribly mutated present.
Fighting with this deformed henchman is more about surviving it than actually beating it. Its primary attacks will be to throw dead guards at you and to charge you; in both cases it takes a moment for the henchman to get ready so use that time to ready some evasive manoeuvres by double-tapping and moving in any direction. When it charges, you can quick-throw a batarang at the mutant () to stun it and allow yourself an opening for a few cheap shots if you like. Just keep avoiding its attacks for a while and, alas, its black little heart will finally give out as it crumples to the floor.
You're a Detective, Start Detecting
Before leaving the room, grab the Riddler Trophy lying in the unoccupied booth and make your way back to the Holding Cells. Should you get lost, just hit to pull up the map; your destination will be marked with a yellow exclamation point. When you arrive in holding, enter detective mode and look around for a flask on the ground. Scan it in and begin following the trail of alcohol to find Boles. Just before entering Secure Transit, look around the reception area to find three more Joker Teeth to take care of.
After Harley brings the elevators down...hard, climb the stairs at the other end of the room to find a Patient Interview tape for Harley Quinn. Head back out to the main part of the room and begin grappling, climbing, and shimmying your way up to the top. Keep on the lookout for a stone slab with a scarab in the centre on your way up. Scan it in to collect your first Chronicle of Arkham. Just beyond the chronicle are five, unarmed thugs. Watch for opportunities to counter their attacks and for chances to finish them off with ground takedowns. With the thugs taken care of, destroy a pair of Joker Teeth on the left side of the room and exit through the ventilation duct on the right.
Escape to Arkham Island
Upon exiting the vent you'll find a group of gun-toting henchmen. Use detective mode and the overhead gargoyles to get behind these thugs without being seen. Once behind them, drop down and quietly crouch-walk up behind the henchmen, one at a time, and use a silent takedown to eliminate the threat. In the control room at the end of the hall is yet another armed thug. Once again, just sneak up behind him and take him down. Leave the control room through the nearby vent.
As you enter the Intensive Treatment Lobby from the ducts, you'll find three armed henchmen roaming the room. The nearest can be eliminated with a silent takedown when he turns his back to you. Take him down and grapple up to the gargoyles to get away before you are spotted. If you have unlocked the inverted takedown move, you can use it to great effect here. Take them out one-by-one without being spotted. If you are found, don't stick around; swing from one gargoyle to another until they've lost sight of you—your armour is bullet-resistant, not bullet-proof. Once you've polished of those three, another wave of three will take their place. Just lather, rinse, and repeat; use the environment and the high ground provided by gargoyles to take them out undetected.
When the room is free of hostiles, drop down into the central office area to find another of Harley's Patient Interviews then get back on the booze trail to find Boles. Once you've found him, check the ground around him to find three more Joker Teeth to dispatch. As you start to walk away, the Riddler will contact you and officially challenge you to complete a rather sprawling scavenger hunt (hence all of the little extras we've picked up along the way). Scan in the portrait of Warden Quincy Sharp to solve this first riddle posed to you. Before you leave, head back to the main office and use detective mode to look for a question mark shape. Line up the question mark on-screen and scan it in to solve yet another riddle.
When you're ready to go, head past the guard into the Utility Corridor. Make sure to stop and check for a radio on a bench on your way out; scan this in to complete another easily accessible riddle. Continue through the corridor and the following vent to escape to the exterior of Arkham Island.