Colony 9 is under attack from the supposed Mechon that Dunban took off back in the prologue but first, Shulk and friends need to get back to Colony 9 as soon as possible. Skip Travel is unavailable at the moment so head to your left and leap off the edge to the water below which will unlock the Making Waves Achievement in the process. Swim to the southeast to reach land again and then follow the trail back to Colony 9 to trigger some cutscenes.
When play resumes, Fiora will leave the party leaving Shulk and Reyn to deal with a lone Mechon as you'll be given a tutorial on how to battle Mechon enemies. Essentially, you'll only be able to deal damage to them when they're under the effects of Topple, this means starting the chain with Shulk's Turn Strike or Stream Edge while using Reyn's Wild Down, what Art you use depends on who you're controlling. Following this tutorial, put it into action against the Mechon M63, a foe that's not deadly and will allow you to get to grips with Mechon battles.
Fiora will rejoin after discovering Dunban is missing and the most logical location is the lab in the Military District. You'll also be given a tutorial on how to flee from battle by selecting the green Flee Icon on the Battle Palette. This is sound advice for now as your weapons will barely put a dent into the Mechon so they're not worth the hassle in fighting them. Instead, choose to take flight and make your way north through Ether Light and then east on to the bridge at Central Plaza to trigger another cutscene. Once you regain control, it's just a short dash to the north to reach the Military District and trigger another scene.
The objective's changed with the lab inaccessible and the mobile artillery in the Residential District your only chance to fight back. Leave the Military District and make your way south toward the Residential District to trigger the next scene as Fiora leaves once again.
You'll be up against three Mechon here and the same tactics apply as to when you fought the lone one back at the Colony entrance, inflict Topple and then make use of your stronger hitting Arts such as Back Slash. With the battle won, more descend on the two and Dunban rushes to their aid, Monado in hand. You'll get a chance to access the Menu before the next battle so head to it and switch to Dunban, the Monado allows you to damage Mechon without having to Topple them first.
This second battle is a lot easier than the previous ones, even if you are fighting newer models, the power of the Monado far outclasses any threat from your foes. Controlling Dunban, when you get the chance, make use of Monado Enchant which will allow Shulk and Reyn to damage the Mechon without Topple. Outside of this, Buster will defeat an enemy in one slice but even simple auto-attacking with Dunban is enough to deal with these Mechon triggering more scenes.
The direct route is now available at this point due to debris so the party opt for the long way via the Commercial District. With Dunban in your party, you can choose to stay and battle the Mechon along the way although there's not much to gain from it. Instead, focus on your destination and dash past the Mechon. Once you reach the entrance, more scenes will take place as Dunban loses his grip on the Monado and Shulk takes matters into his own hands.
You'll be thrust back into battle and another tutorial will inform you that Monado Arts has replaced his previous Talent Art, Turn Strike. When you activate this, this will change to a different Battle Palette full of powerful Monado based Arts. You still only have access to Monado Enchant and Monado Buster for now but like all Monado Arts, they're highly effective against the Mechon. Begin the following battle with Monado Enchant to allow your allies to damage the Mechon and then use Monado Buster on any survivors.
Tip: Take a look at the Arts menu following this battle, you can now level up the Monado Arts by selecting the icon in the Battle Palette. These are usually more costly, especially Monado Buster but if you have the AP available, it's worth increasing Enchant's level.
When you're ready, continue to follow the trail to the south to reach the Residential District to trigger a showdown with the faced Mechon.
Boss: Metal Face
This hulking monstrosity is virtually impenetrable, even with the Monado, at the start of this battle. You can do little more than simply swing away at it until a series of devastating cutscenes take over.
When you regain control, you'll find yourself in battle once more with Metal Face. Your attacks will still be deflected but you'll learn that the Topple combo will allow you to inflict damage. As the tutorial mentions, it can be hard to do this while deflecting constantly and this is where Chain Attacks come in. Activate one as soon as you can and use the Arts to inflict Topple on Metal Face, allowing you to deal damage.
You'll still want to make use of Monado Enchant to increase the damage output of Dunban and Reyn and then use Monado Buster when you get the chance again. Metal Face only has three real Arts available to him, Killing Claw will deal heavy damage to the party member while inflicting Strength Down, Last Resort will deal less damage but hit twice and Daze the target while Dead Way is an Ether based attack that will knock you back and output similar damage to Killing Claw. Your options are still rather limited here but keeping up Monado Enchant and unleashing Buster when available will bring it to 50% HP where the battle will end.
More cutscenes will take place, showing the aftermath of the Mechon Attack, setting up the motivations for the journey ahead and ending the chapter.