|| After defeating Dark Erde Kaiser, go to Heaven's Ruins and proceed to where you fought Nepos Rigas. Approach the purple stuff to fight Mad Skelter.
Boss Battle - Mad Skelter
When the battle starts, Mad Skelter will mimick the parameters of the highest level character in your BATTLE PARTY. suggest bringing MOMO in -- making sure that she has a higher level compared to your other party members. MOMO has the lowest STR parameter among all your characters, but she also has the highest EVA value. Despite this, I believe it'll all be worth it in the end.
Most of the boss' attacks are physical in nature, so since MOMO has low strength, Mad Skelter's attacks won't hurt you at all. Thus healing is hardly required. "Inescapable Arrow - Final will still kill you though, and your attacks will miss a lot. Cast Flame Sword on your characters, then move one of them to the back.
Start unleashing combos afterwards, hoping that they hit. Once you manage to bring its HP down to at least 26,000 or so, bring in Shion, cast Flame Sword on her and summon Erde Kaiser (while the boss is downed) to get this fight over with.