You'll have to kill probably between three and five guards to buy enough time. Soon, the panel will open and your friend will poke his head down. When he reaches an arm down, make sure you have a shield constructed for safety then stand beneath his arm and he'll pull you to safety.
Back in the open air, your friend sets you down beside the hole. Below, the guards shoot up, loosening rocks above to fall through the hole onto them. Your friend runs off to the right. Run to the left side of the screen, then run right. Gunshots come from inside the building to the right where your friend went. By running left first, you are now on a back ledge that takes you around the building instead of through it. Run to the right and you'll pass behind the building, visible in the windows. Inside, the guards have your friend at gunpoint. When you pass the building, run left inside and kill the right guard.
If you're fast enough, you can use the guard's own shield for defense and blast him before he has time to erect another shield. Meanwhile, your attack has given your friend the distraction to attack the left guard in a fist fight. You MUST kill the right guard before your friend finishes off the left guard or else the right guard will kill your friend and you can't finish the game without him. When both guards are dead, run with your friend to the right until you reach the ledge. Wait a few moments until your friend beckons you to the edge. Walk to the edge and he'll throw you over to bounce off the awning on the right side, landing on the opposite ledge. You have to let him throw you across because you can't make the jump on your own.
Next your friend tries to jump across but he's too heavy for the awning and winds up hanging precariously from the end of it. Make a standing jump to the left and you'll grab onto the edge of a lower awning and swing down to a passage to the left. In this passage, two guards will run out to shoot you. They're tough to beat. My technique is as soon as I land in the passage, I run a few steps back to the right, then turn, crouch, and make a shield. Then I super-blast the first guard's shield then shoot the guard. At this point, the second guard takes a few steps forward and erects a second shield. If I hadn't taken those first steps to the right, he'd be able to reach right past my shield and shoot me. Now make a new shield, super-blast his shield, then shoot him dead. When you've finished off both guards, run to the left and keep going through the next room.
Keep running to the left. In the room with the staircase, keep running or the hidden guards will shoot you. Don't go up the staircase yet. Keep going to the left. In the next area, three unarmed guards run away from you. Don't shoot them. Stop for a second and you'll see one of the guards get trapped behind a door and a gate. Walk up to the door until it opens, then crouch and point your gun at the guard. He'll raises his hands in surrender, then reach up and push a blinking red button which is apparently the emergency shut for the doors in this room. Now go back to the stairs and head up. At the top of the stairs is a guard behind a door.