Disgaea 4 A Promise Unforgotten: X-Dimension Guide Chapter 7 Pt.1
Stage 1: The level is separated by two different colors of Lonely Panels. Keep a unit on each side and kill the enemies. If you're feel the need to have more than one unit on each side, just destroy the Lonely Blocks.
Stage 2: Someone with a throw range of at least seven is necessary. Also a character/s with a jump of 48 will make your life easier so you don't have to tank the damage from the exploding barrels. You have seven turns to reach the end area and toss someone over to kill the Prinny. Careful as I landed too close to the Geo Block and the Prinny attacked it, nearly giving me a heart attack in the process.
Stage 3: Meh, its a level of nine Reapers and you're basically confined to a stage of Lonely Panels. Just hit them one at a time. I don't recommend standing on their panels as you'll take damage. Don't forget the Reaper at the very top of the staircase.
Stage 4: You need a monster and human to traverse their respective sides. You can destroy the Geo Blocks that may hinder your monster/human units if you want. I didn't feel it was necessary.