Shurelia (boss)
Shurelia during her possession by Mir, and the first phase of the final Phase 2 battle. Given her position as the Tower Administrator, this means that she has all of the resources of the First Tower at her disposition to attack the party, and will not hesitate to use them, while also having a very high defense, speed and elemental resistances thanks to her Linkage armor.
Her skills are as follows: Satellite Strike, in which she uses the Symphonic Power from the Tower to deal large magical damage to a single target; Satellite Rain, in which she does the same to deal large magical damage to all the Vanguards, and two Stored Attacks. One is a Lv. 1 one called Primal Word, her most basic Song Magic, in which she launches a ball of energy to cause average magical damage to a party member; but the other one, which she uses if the battle has been drawn out for too long, is the real danger: a Lv. 3 one called Ar tonelico, also known as the ultimate Song Magic, in which she launches a large amount of the Tower's Symphonic Power to the party, causing extremely large magical damage that ignores defenses. As you can imagine, this attack will most likely leave your party almost dead, and if you fail to block even one of the target rings, it'll undoubtedly kill off your Reyvateil.
Due to her Linkage, you will have to depend on non-elemental Song Magic like Slash Sword, Void Slasher, Flower or My Hero in order to take her down. If either Aurica or Misha have learned their own versions of Ar tonelico/Tower Connection, even better, as you'll be able to defeat Shurelia before she even attempts to blast your party away with it. However, keep an eye on the HP of your party, as if you don't keep yourself healed, Satellite Rain will most likely destroy you. Due to this, make sure that your party members have a minimum defense of 1700, and try to have the lastest equipments for your party in order to withstand better her attacks. However, don't use up all of your Song charges and items, as you'll still have one more battle after this.
Trivia
All of Shurelia's skills have a flavor description in the game's files, although they can't be seen in the game proper. Additionally, it shows that she was supposed to have an additional skill called Full Cushion, which has the same effect as the Song Magic of the same name she can use while in the party: nullifies all damage dealt to her as long as its kept active. However, for unknown reasons, Full Cushion was dummied out.