Actually 4 (active) Summon Gates, and you have to destroy them all to spawn a Boss. You can guess what you have to do after that.
Nothing much to say about this mission except that it's easier than Battle of Changban. It's a regular straightforward mission with 'go there and kill stuff' objectives.
This introduces the timed-event missions where you are supposed to wait for certain times to accomplish objectives. There's nothing you can do to speed things up, you will have to wait.