UNDERDEEP elemental weakness is a property of the individual monster, not of the dungeon. The game has six elements - Fire, Water, Earth, Wind, Light and Dark - and the character panel keeps a counter for each one, while an AOE weapon deals extra damage to any enemy that is weak to its element.
Underdeep elemental weakness: the six elements on the panel
Fire, Water, Earth, Wind, Light and Dark. Your counters sit on the character panel next to the attributes, and they tell you which of your weapons have an element and how often you have used it.
Which monsters are weak
- In Dungeon of Gold (Upper), both the level-9 goblin and the level-9 mimic are weak to fire.
- In Dungeon of Gold (Lower), the monsters carry their element on the name: bat (fire), rat (wind), wolf (water), orc (water), troll (wind) and mimic (fire).
Weakness is not a resistance system that applies to every monster: only the enemies listed as weak are documented as taking extra damage from that element. The health behind each of these monsters is in the monster HP tables, which is what tells you whether the extra damage is worth building for.
Why AOE benefits from elements
Elemental weakness is one of the main reasons to use AOE. An AOE weapon deals extra damage to enemies that are weak to its element, and skills can boost the element of the weapon you are holding - which is why the strongest setup is two AOE weapons that boost the same stat and share the same element.
That also means your element choice is a build decision rather than a flavour pick, so it belongs with the weapon and stat order in the build guide.
Element skills need a matching weapon
Element-boosting skills are only worth taking when they match the element of an AOE weapon you actually use. Bought without a matching weapon, they occupy a slot and do nothing, so hold off on them until the weapon is in place.