Claptrap will lead you from the meeting place over to the doorway leading to Hero's Pass. He will go over to the door and attempt to get it to open.
The first wave of loaders is simple: GUN Loaders mostly. Take them on out. This doesn't go according to plan as the secondary door is closed and Loaders are called in.
The second wave of loaders start with EXP Loaders right by Claptrap. Move away and take them out. They are followed by HOT Loaders and more GUN Loaders. The second wave ends with Claptrap activating more Door defenses: the Door Turrets.
The reinforcements will run out at this point and Claptrap will get the door open. He is greeted by the sight of numerous stairs in front of him.