Makai Senki Disgaea 2/Walkthrough/Episode 3
(3-1) Poisonous Swamp
Make sure to have an ally that can hit a target from 4 panels away before attempting this map, if you want an easier time at it that is. The Zombies on the field won't move until you get close enough to them, so you should probably focus on dealing with the encroaching Ally Damage 20% effect first. The Null Geo Symbol that harbors the damaging effect is off on an island near the edge of the map. That is why you need the 4+ panel attack range ally to destroy it. After that, just mop up the Zombies one by one.
(3-2) Dangerous Refuge
This map introduces Geo Monsters, small monsters that move Geo Symbols around between yours and your enemies' turns. None of the Geo Symbol effects on this map are harmful, so they can be ignored. You should instead focus on dealing with the nearby Moths quickly, then stand your ground as the rest of the Moths rush to attack you. Having a Healer with Espoir will help deal with the Moth's poisoning attacks.
(3-3) Corpse Garden
The map is populated completely with Slug monsters, so leave ice magic users back at base. The major thing to do with this map is steal the Dark Rosary item from the lone Vodianoi monster, because it is a nice accessory and because the monster will not be carrying it on future visits once this map has been cleared.
To start with, this is supposed to be a hopeless battle. Once 10 of your allies are defeated, the story will progress, so make the best of this situation in the meantime. Bring along 10 Thieves (their levels/stats don't matter for this) and try to steal Etna's Testament item. Yeah there is only a 1% chance per attempt to steal it, but with 10 Thieves, that becomes a 10% chance at getting a powerful item that will serve you well throughout the rest of the main story. This is also a good opportunity to learn the nuances of save scumming, in order to obtain a Legendary class Testament for all your hard work. Ignore the treasure chests, they are not anywhere near as valuable as that Testament. If you manage to defeat Etna during this fight at any time in future cycles, you will get an alternate bad ending after the battle and then be forced to start a new game cycle.
When you revisit this stage after Etna leaves, you'll find it filled with Moth monsters and nothing else. Bring a Healer that knows Espoir to deal with the poison attacks from the Moths, and bring some long-range allies to route the Moths, as the map's layout makes navigation awkward for short-range fighters.