Super Dungeon Tactics

Unlocking this achievement is simple once you obtain a staff (or two) that do Tile Fire damage. These staves, when used to attack, cause the squares they damage to also light on fire for two rounds. Depending on the size of the damage area, you may need multiple heroes to wield one; the 3x3 damage areas the smaller staves do are not enough to light the required 25 tiles by one character in battle.

Simply equip your characters with these staves, and use them in battles, making an effort to always cast your damage zone on un-lit tiles that do not have scenery in them.