Sleipnir
A monstruous dragon that lives in the Blastline, this creature feeds off from the plasma that forms in said place to attack, and is famous for being the main cause of the Platina airships crashing. This monster has also fought with Garzburn over the supremacy of the Dragon's Nest for quite a long time. At the start of Ar tonelico: Melody of Elemia, it was the culprit of Lyner's airship crashing into Viola Forest, and Lyner and his friends had to defeat it later on in order to cross the Blastline and arrive to the top areas of the Tower, in pursuit of the Mir-possessed Shurelia. On the other hand, in the OVA, Sleipnir was made into the main villain, which was defeated at the end by Lyner, who was powered-up by both the Harmonics phenomenon and Aurica's Moiskyrie Song Magic.
Battle Strategy
Sleipnir's reputation preceedes it, as it has quite high stats, it's nigh immune to all elements, and if fought incorrectly, it can easily wipe out a fully healed party. Its skills are as it follows: Crushing Bite, which deals large physical damage to a single target; Demon Breath, which causes damage and Paralysis to all Vanguards; Limit Release, which increases all of its stats; Fill Up, which reverses all of its mode shifts and restores its HP; and finally, Wave Cannon, a Lv. 3 Stored Attack that causes large amounts of Thunder-elemental damage.
However, it has a fatal weakness: if attacked with a Thunder-elemental Song Magic, skill or item, it'll trigger its Dormancy counter (also happens whenever it uses its Wave Cannon attack), which puts it to sleep for several turns and lowers all of its stats, allowing you to attack him as you see fit until it uses Revival, in which it'll return to normal. Try to use either a Quicklezz/Quicklezz RX in order to reduce its speed, so it remains in Dormancy status for a longer time, and you'll easily defeat it.