The last boss fought previous to the game's final battle, this fight combines all of the three A.B.R.s into a single body, divided in three parts. Also, you will have a set party of Lyner, Jack, Radolf and Shurelia for this battle. All of the A.B.R.s had a massive stat increase, and while they lost several of their individual skills, they gained a nasty gimmick: whenever any part is damaged, the α part will heal all of them, while the β part still keeps its annoying ability to absorb Song Magic. Finally, after the β part has absorbed any magical attack, the γ part will change all of the A.B.R. parts into Delete Mode, which will then begin charging its Lv. 3 Stored Attack: Eraser Wave, which deals large damage.
However, it does have a weakness: when the A.B.R. changes to Delete Mode, the β part won't be able to absorb magic, so make sure to take advantage of this chance. To eliminate it, make sure to first use a Blue Magic to buff your party, and then use Break Blade, Divine Punishment/Eternal Rain and physical attacks to annihilate the β part, so you can freely use magic to attack and disable the Eraser Mode and the Eraser Wave. After it, attack the α part to get rid of the healing. After both of these parts have been destroyed, the γ shouldn't pose a large threat, and you can easily defeat it.
This version of the A.B.R. is also fought in the first chapter of Shurelia's Binary Field, and while your party for that battle will only be Shurelia and Lyner, the A.B.R. was severely weakened and can be easily defeated. Winning against it will net you the ability to summon it to deal Fire and Ice-elemental damage with Shurelia's A.B.R. Song Magic.