At the NE corner of G-10 you will need to use one Giant Scale. The displacement will grow in size and will become usable to proceed to the next section.
Once an Unstable Displacement grows it will stay usable for approximately 3 minutes.
Go to (E-10), where another Giant Scale will be needed for the Unstable Displacement.
Go forward and stay to the right to enter the Spatial Displacement at the NE corner of (D-9).
Head straight in this area to reach another Spatial Displacement at (E-8).
You should see it immediately.
If you get the dialog which starts "You can sense the majestic presence of a terrestrial avatar...", you've got the correct one. Use this one to enter Monarch Linn.
Next head forward to the Spatial Displacement and enter the battlefield for Ancient Vows.
To save the time used by the cutscene that will play once inside, one can run away and enter once again, having then the option to skip the cutscene. Only after leaving the area without successfully clearing the battlefield will the cutscene become mandatory again upon entry.
Upon successfully clearing the battlefield, everyone on this mission will be teleported to South Gustaberg. Those who have completed this before, will be teleported outside the battlefield.
They are capable of changing Jobs at will. The job being used (and the special abilities they can use) can be identified from the weapon the Mammet has equipped:
"Hand Form" (Initial form, no equipped weapon; seems to shoot projectiles from its sleeves. Moderate attack speed.)
Transmogrification: Absorbs all physical damage for ~30 seconds. Effect also applies to physical weapon skills, and even abilities like the Eagle Eye Shot. Extremely dangerous. Everyone should have a macro to call out when they see Transmogrification so people can halt their attacks as fast as possible.
"Sword Form": Warrior-type Geared toward Weapon Skill-like attacks and TP-based AoE status abilities. Extremely fast attack speed; Utsusemi is hard to maintain against this form.
Yellow Liquid (obtained from the Mammets and sold via Auction House under Medicines) will lock the targeted Mammet into its current form for thirty seconds, making it unable to change its job. This is useful for both kiting and tanking. Kiters can start by locking their Mammets into "hand form", though it's bad for a Mammet being tanked because of Transmogrification.