Once inside, go up the stairs and at the top, take a right and look through the giant glass window into the center of the building. After a cutscene, go right and you should see a ladder. You can climb the ladder... or you can just use your grappling hook. Get to the top and Batman will mention taking out the guard with the gun. Hold the right trigger to crouch and close in on the guard. When you are behind him, hit the counter button to do a silent takedown. Once he's down, look down and stand by the edge. You should see the glide kick icon. Pick an enemy and hit the strike button and you'll automatically drop down to attack.
This fight isn't really as bad as it looks. Most of the enemies actually flee leaving you to fight what is probably a smaller group than you had outside. Keep your wits about you and you'll do fine. After you finish the fight, watch the cutscene.
