The Aurum Vale is a level 47-49 instance located in the Coerthas Central Highlands. It is unlocked by Nedrick Ironheart in Vesper Bay, and is not part of the main story line. Level 50 players will be synced to level 49 for the duration of this dungeon.
First, a note: the various Marbol fruits that are located around the room are of key importance during this fight. Each person should position themselves next to one of these: the tank at the one next to the boss, and the healer at the one directly to the left of the entrance to the room. During the fight, the boss will cast an AoE debuff called Gold Lung Blast, inflicting Goldlung, that will tick for moderate damage for each stack acquired. This cannot be removed with Esuna. At 2 stacks, each player should eat the fruit; it will respawn in time to pick it up again later.
This fight is aggressive for healers, who will be pressed to keep everyone's health up as the debuff begins to stack, but otherwise there are no major abilities to note.
Coincounter has three melee abilities: 10-Tonze Swipe, which is an instant frontal cone; 100-Tonze Swipe, another frontal cone that will usually reduce a tank to half health, and one-shot healers and DPS; and finally 100-Tonze Swing, which hits hard enough that it may wipe an entire group. Stay behind the boss to avoid 10- and 100-tonze Sipes, and run from 100-Tonze Swing.
During the fight, Coincounter will repeatedly use an aggro wipe that will erase his aggro table and cast purple electricity at the party. When this occurs, DPS must hold back so the tank can get the boss back in control.
Finally, the boss will also randomly turn towards a random party member and cast an AoE beam towards them. This can be interrupted with stun attacks. There is no range indicator on this, so simply run to the side of the boss turns towards you. Avoid the abilities of this cyclops, and he will fall soon enough.
The first phase of this boss fight is very similar to the first boss, with two very important differences: One, the stacking debuff the Mistress casts does direct damage instead of a damage over time effect. Second, the fruits used to dispel the effect do not respawn anywhere near as fast, and so players will need to stack 3, or even 4 (if the healer is able to cope) stacks before eating the fruit each time.
The second phase can be tricky, depending on what kind of DPS you have in the group. Enemies seen earlier in the instance as trash mobs, Morbal 'Fruit' (a trap! These turn into Morbol Seedlings if not killed quickly!) will begin to spawn. They will be similar to the trash mobs seen before, but will now apply a stack of the debuff with their auto attacks. If too many pop, and stay alive, they will wipe the group. AoE DPS is best suited to destroy them, so they should do so quickly. DPS will need to alternate between the adds and boss to finish this fight as soon as possible before the healer's MP runs out.