Walkthrough - Arkham Island #7 BAA Guide

Exit the Arkham Mansion building and proceed to the north-west, towards a well-known passageway. Once you're inside the tunnel you'll discover that the surrounding area is heavily guarded. Start off by attacking the only inmate equipped with a stun-gun and once he's pacified deal with his colleagues. Proceed towards Arkham North when it's safe.

You should soon hear a sniper warning and I wouldn't recommend ignoring it. Grapple up to the nearest guard tower. You'll now have to wait (while hiding behind a cover) until a nearby sniper turns his back on you. Use your Line Launcher quickly to get to the roof area of the Intensive Treatment building (screenshot #2). Land on the balcony and eliminate the sniper before he fires his rifle. Once he's gone choose a nearby passageway to travel to Maintenance Access.