Ayatane (Ar tonelico Qoga boss)
Having received power from the Antibodies in exchange for creating Finnel, who would be used as a vessel for Ar Ru; and using his own powers as a member of the Teru, Ayatane Kureha transformed into a dragon to kill the party at the Tower of Origin as revenge for disrupting his plans to eliminate humanity.
Although large and imposing, which of course gives him a rather high Attack power and Defense, Ayatane has a terrible movement speed, which coupled with the fact that his attacks have a rather long start-up animation, should allow you to block and avoid them. However, don't let your guard down, as one of his attacks is a wing spin that will hit everyone around him, so try to get away once you get the signal that he is going to use it. Finally, be very careful with his Stored Attack: Mega Flare, which aside of causing great damage also has a drain effect, which will give him back every single bit of damage dealt to your party, further prolonging the battle.
The best you could do to defeat him would be surrounding him with all your party members; and continue attacking him and healing yourself until your Reyvateil has Purged far enough and you have at least 90000% Burst, which will allow you to unleash all the Ultra Supermoves of the party in rapid succession, most likely defeating him. If that isn't still enough, use either a Lv. 4 Song Magic or the Flipsphere, which should finish him off for good.
Trivia
This version of Ayatane is most likely a direct reference to the Final Fantasy series' iconic summon Bahamut, most specifically, his iteration from Final Fantasy X, as Ayatane's battle stance is identical to the one Bahamut has in said game, and the animations they both have for using Mega Flare; which has been Bahamut's signature attack since Final Fantasy III, are pretty similar as well.