The last phase of the Phase 2 final battle, this monster is Mir's manifestation as a virus, and it has terrifying power. As your party will most likely be pretty hurt after the previous battle with Shurelia, you'll have to take care, and make healing your highest priority, as Demon Mir has a higher attack power and HP than her.
Its skills are the following: Kill, which deals large physical damage to a single target; Power to Destroy, which attacks all vanguards for large physical damage; Break All, which deals extremely large physical damage to a single target; and finally, a Lv. 3 Stored Attack called Circulate, which causes extremely large physical damage. As you can see, its skill are all pure raw physical power, and if you don't keep your party healed, most likely you'll end up defeated.
The only consolation on this is that Demon Mir has a lower defense than Shurelia, so if you can hit fast and hard enough, you shouldn't have much problem to take it down. Make sure to equip your best items and Grathnodes prior to entering the Rinkernator's final room, and also Install the best red and blue crystals you have into the best song your favorite Reyvateil has. Also, make sure to come stocked with several strong healing items and MP restorers.
Other Appearances[]
Demon Mir can also be summoned by Shurelia after defeating it in the fourth chapter of her Binary Field. While during that battle you will only have Shurelia and Lyner available, it will be severely weakened, so you shouldn't have much problem in defeating it.
Likewise, and although it doesn't physically appear in it, Jakuri can summon it in Ar tonelico II: Melody of Metafalica with the Demon's Dance Song Magic gained from her Lv. 5 Cosmosphere, which also requires defeating it. While that will be a Reyvateils-only battle (given your party at that time will only be Luca and Cloche), its extreme weakening means you shouldn't have much problem either.