That part is easy once you discover the 'pump fake':
The trick is to go up and wait for him to come up. When he starts going up, go back down. Once he starts to follow you back down THEN you have time to get by him.
The trick is to go up and wait for him to come up. When he starts going up, go back down. Once he starts to follow you back down THEN you have time to get by him.