Disgaea 2: Cursed Memories/Town Fields

Stage 1-4: Dead Tree Field
Geo Symbols - Aqua panels only
 * Aqua symbol 'No Lifting' on Aqua panels
 * Aqua symbol 'Reverse Damage' on Aqua panels

Dark Sun - 10 Difficulty
 * 1) Ally Changed into Dragon
 * 2) (repeats)

How to Unlock: Kill either an enemy or ally Green Skull on the normal 1-4 stage. Transportation to the Dark World version is instantaneous.

If you have one or two Healers, or other characters with healing spells, of decent strength, this stage should be no problem for you. All eight magicians are lined up in a 2x4 pattern on the Reverse Damage Geo Panels, and will not move from their positions, so you should be able to take them all out in two Giga Heal area-of-effect spells.

If you somehow don't manage to defeat all of your enemies in the first turn, or if you only have one Healer, it probably won't hurt much to just keep her out there to endure your enemy's attacks. Healers have naturally high RES, and you've likely focused her armor to enhance RES since it's the stat used for healing spells. Add that onto the fact that all Healers have an innate ability to resist elemental attacks, and you'll know your Healer is sure to stay alive.

The Ally Change will only affect one of your human characters on the map, chosen at random. Since healing spells are kept with the change into monsters, there's no need to worry about Healers.