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.
One of the longer TBSs but it's fun to do this once in a while. With 4 Bosses, 3 Summon Gates, several minibosses and hordes of mobs, it has its fun moments.
Sounds like a lot of hassle but this mission is pretty easy if you know what to do. It also gives up to 50m+ EXP per merc which is pretty high for a level 132 TBS, and 14 reward boxes is more than the usual.
This mission is time-based which forces you to spend at least 23 minutes on it, but 15 reward boxes in the end kinda makes up for it, specially if you can solo.