Once Bentley meets up with you, immediately go to where the statue is posed on the ground and knock it out of the way so Sly can stand on the pedestal and pretend to be the statue instead so you can surprise attack the incoming bird guard. Then, go with Bentley by the cobweb so you're up on the bridge.
Here, you must pose some more by pressing O so you can surprise attack at least one of the guards as they go by. When Bentley mentions that he is done setting up a bomb, hurry down to his side so you can have enough time to defeat the guards that come.