If you never heard of this game before you can check my Nine Chronicles Review / Overview

If you have just started the game and need the basics covered, please read my starter guide since we are not going to repeat that information here.

I believe that the fire dungeon is not worth the effort after the release of world 5 (stages past 200). Those items are way stronger and don't cost NCG to craft. So until these items are reworked to cost less I'd avoid it.

General info

Mimisbrunnr is the first of the elemental dungeons, and so far the only one.

It requires the player to have finished Stage 100 and to have only fire gear equipped.

It is not however a place for new players to jump at 100 and it's actually endgame content since from half of it the monsters have level 205, the highest in the game right now.

The maps give no experience, and the rewards can only be used for crafting fire recipes: they are Fire Tier 1 (white) and we have one for each slot.

The fact that they are Tier 1 makes them even better since they are the cheapest items in the game to upgrade, and the fodder for them only takes 1 block to craft.

When to start fighting in the fire dungeon?

If you got level ~150 and NCG to risk on the crafts, it can be an interesting strategy to invest in gear and farm yourself some fire necks - you only need to be able to beat stage 5 and that should be possible with decently rolled +3 T3 (blue gear) and a mediocre war belt (or a dungeon belt that you can craft yourself, but the war belt is cheaper probably). 

Otherwise, I suggest you completely ignore this part of the game until you are level 200 and can get the bronze badges from the arena. You will miss too much if you are lower level and you won't be able to craft the top recipes anyway.

Also, take into consideration that most of the items that you will build might sell for less than the cost of the crafting itself... but the best rolls are still very expensive and needed... so it's a gamble :)

Crafting - fire dungeon recipes

TL:DR The top rare rolls are the best in all slots except the belt (which is close in performance and cheaper than the war belt, so quite good anyway).

All the crafts cost 300 NCG except for the sword that costs 450 NCG.

There is a tab with the fire gear detailed in this document.

Gearing up to beat it

If you are not in a hurry, and you are doing this after level 200 (which you should since without badges there is no chest and sword crafting), you can self-craft your way through the fire dungeon. Or if you want to get over it fast and start crafting all the recipes you can just buy the gear explained instead of spending a lot of time and NCG to craft it.

1. Craft yourself / buy some good rolled T3 (blue) Fire Sword, Chest (or buy a T1 dungeon chest if you find it cheap), Neck, 2 rings, and if you can a war belt (spd is the best but anything works). Making them +3 should be enough. You can make everything faster if you buy 2 Dungeon Fire Thin Rings from the start since they are the last to craft.
2. Beat fire dungeon up to stage 5 and craft yourself a great neck - might take some tries, attack + hit is the best combo. Make it +7 since it's the best in the game anyway.
3. The neck should take you to stage 11 and there you can craft yourself fire chests until you get one with a ton of life. Speed + big hp is the best chest in the game right now (and there are only a few in crafter so far), but a lot of health is great too. Make your best chest +7.
4. Push it with food (if needed) up to 13 and farm yourself a spell sword. Almost all swords get a spell, so they are all great. I prefer one with Flame Blow Attack (single target but a high chance) and attack instead of a double spell. You can get the AOE (area of effect) Magma Tsunami from the ring.
5. +7 the sword and that's it, should be enough to beat it up to rings.
6. Craft yourself a ton of rings until you get 2 that are great and beat 20 too (no special reward past stage 17 btw)

As a reference, way different than the build in the video from the start, here is the "budget" gear that helped my second character defeat fire dungeon 20. It has a decent spell sword, a great pure health chest (18k hp..), a great roll on the neck (but only +4) and the rest of the gear is nothing special. You can of course get along with worse rolls that are upgraded to +7.