Level 3:
The guard will throw you immediately. As you're falling, click on the ledge to grab it. Click on the guard and select the pistol option. Pull yourself up by clicking above the ledge. Click on the dead guard to pick him up. Click on the spiked area to throw him down. Click on him once he's down there to jump on him and avoid the spikes. Click on the opposite ledge to jump across. Enter the doorway.