XenobladeX Mamma-Mia!

Requires: very long. It involves several quest chains and ends with enemies that’ll require your near endgame level of preparedness. First thing’s first. Leave BLADE Barracks and take a right towards the east gate. Look for a girl name Suriella. Talk to her to start one of the mission chains. There are 3 mission chains besides Mia’s chain. Some can be completed in any order except Miracle Maker and Rites of Ruin so no need to specifically follow this, as this just lists where they should be. If they aren’t there change the time to day or night, or you may need to progress the story.
The first one is from Alex who is at the western C.D. in front of Army Pizza, mission called Alien Nation. Follow through with the mission chain until you start getting missions from Eliza and Alice, where you’ll get Shotgun Diplomacy, The Line of Fire and Alex’s Last Stand.
The second mission chain comes from a Professor who is at the far north area of the I.D, called Mystery Man. That mission is followed by a mission given by Veven called Professor B’s Return.
The third mission chain is a lengthy one that should start at the A.D. upper level near BLADE HQ, called Attack on Biahno Hills. Another mission is accepted after chapter 5 from Ajoa who stands outside the east side of the water plant in the R.D, called Water Woes.  Also after chapter 5 should be another mission waiting at the church in the R.D, called Miracle Maker. Later on in the story is the conclusion to the mission that started at the church, called Rites of Ruin.
Next is a mission that may start later on called Good Fortun which would start in the area of the I.D. from the southwest of the west gate. Once that mission is done, Definian Downfall can be accepted after Chapter 11. Starting at the centre of the C.D., look for anyone on a rooftop or parking area. This should be obvious and will be the quest giver. Once again, be prepared for a difficult fight.

First, after Chapter 3 accept Mamma Mia normal mission from Mia standing near a crane up north and to the west of Division Drive. A way to get there without platforming is to start in front of BLADE HQ, and go north where the path narrows. Hug the building until you can take a left and follow this straight up to the crane. Accept the mission and follow through.
Part 2: complete Chapter 5 and meet Mia atop Noctilum’s Seabound Coil Tree which is northwest of FN Site 220 which is at the area where the small and large regions of Noctilum join. Go inside and ascend the coil. Accept her mission and follow through.
Part 3: Complete Chapter 6 and go to Oblivia’s Leaning Ring which is just east of FN Site 307 in the southern region of Oblivia. You can go towards the Giant Ring in the centre of Oblivia and go south of it towards the lake. As you approach the ring you should get a scene to load. To be safe, approach the western side of the ring. Follow through with the mission.
Part 4: Complete Chapter 9 and “A Girl’s Wings” affinity mission for the flight pack. You can find Mia at the Cleansing Spring where the Nopon Caravan is. Around the southern area of Sylvalum is the Noctilent Sphere. On the segment to the right of it is where the Cleansing Spring is. It can be reached by going east from the sphere while hugging the right side.  Eventually the wall ends and you can take a right and head into the Cleansing Spring. Meet with Mia, accept the mission and follow through.
Part 5: Complete Chapter 11, the Definian Downfall normal mission and survey 65% of Cauldros. Starting at FN Site 505, go north across the lava pit and go into the cavern entrance. You’ll have to come back into the Ganglion Antropolis and if you recall the area after the Shrads, there may have been a fork in the path where there was a barrier. Go in and defeat the Definian if you must. The goal is the shed where Mia should be. Follow through with the mission and she should be ready to join.

Definian Downfall: This area is home to enemies easily above level 50. Search for 3 keys located at: when you enter there are two enemies. On the left behind one enemy is an elevated area where one key is located. From the entrance hallway, take a right down to where an enemy Skell is sitting. Go past and stick right and go up. The enemies that appear should have a key on them. Finally, from the entrance, go right and take a left across the bridge towards the area of progression across the area. If you can avoid the enemies in any way you can think of, go ahead. Once you’ve made it across and go through you may see a barrier. Ignore it for now and proceed left past the Shards. Further past are more enemies. Keep making your way as far in as you can to find the last key which may be in a chest or on an enemy.  With all 3 keys, go back to the previously mentioned barrier and go through. Prepare for an on-foot battle, followed by the Skell-capable battle outside. With all this done, the mission should be complete.