The group setup we had in our first attempt consisted of 4 clerics, 2 chanters, 1 templar, 1 gladiator, 1 SM and 3 rangers. If you have attempted this boss with a different group setup that was efficient, please let me know so that I may add it in here.
Other things to keep in mind and that are extremely important is the terrain you are fighting in. The following picture picks out 3 different spots on the map. These spots have permanent portals, and if a member of your group would click on this portal, he would be teleported to the top, where Yamennes spawns a gate which then spawn adds. These gates must be priority for the DPS and must be taken down ASAP!
Now that we know there are 3 gates, the alliance of 12 members must divide itself accordingly so that at there are at least 2 dps per gate in order to take it down efficiently. Once the groups have been setup (total of 3 groups) each group with their clerics must stand on TOP of the portal! This is extremely crucial because of the following?..
Inflicts heavy damage on the target and all enemies within 6 m of the target, and stuns them for a short while.
When Yamennes is about to cast this spell, it will select a random target and start casting it, the templar must call it out in vent and that person has a total of 4 seconds to run away from his group. If he does NOT run away, the spell will be casted on him and it will spread to the nearby players, hitting 6K dmg per person on his group.
Where would the person run? Look at the following picture;
If everyone is standing on top of the red circle(portal) and not spread out, the person being targeted with Crystal Collision now has more empty space to run to so that only he may get hit. So during those 4 seconds, that person must move onto the green area as shown in the map, away from his and other groups. If done correctly, only that 1 person and the templar should get hit for 6k damage.
These gates will spawn every 30 seconds. They will spawn ads which will put a reflect on the boss if not quickly killed. The first gate of the fight, will spawn at the bottom, and like said before, at least 2 dps per gate must be dpsing gate. The 2nd gate will spawn at the top, so players must take the portal to destroy it, and once dead, jump back down to dps the boss.
This spell Inflicts heavy damage on HP and MP of all targets in front. Meaning that, it CANNOT hit anyone in the alliance at all except for the templar. This being said, the templar must face Yamennes away from the groups (Yamennes should be facing 1 of the green spots highlighted in the map). If done right, it will minimize the chance of anyone getting hit by it.
When the boss selects a target, and casts this spell, it will paralyze said target for a while. In addition, the target will receive continuous damage and it CANNOT be dispelled. (I am not sure if this skill is AOE or only a target spell)
Last but not least, we have Resistance. This is an instant cast that will make the target bleed but have some resistance to certain physical attacks. This condition cannot be removed by any means.
Agro is random. Enfeebling Burst didn?t work. Trick is survival, not DPS. Boss uses skills that paralyze, stun, and deal massive damage to HP and MP.
DPS have a set of weapons with attack and/or crit socketed. (Not sure if this will help on gates.)
All players have a PvE gear set with full HP. Use food items that increase HP.
Chanters use as many of these skills as they have available as often as possible:
Aetheric Field or Ancestral Aetheric Field
Blessing of Stone
Curtain of Aether
Word of Inspiration
Word of Protection
Word of Spellstopping
Try having healers use full healing stigma tree for Benevolence, which increases healing boost, but also increases mana usage. Make sure to have plenty of mana recovery skills & items handy. Also use a PvE set that increases healing boost socketed with all HP.
All casters and ranged attackers should be max distance from their target!
Keep alliance in 2 groups so all members can benefit from group heals and chanter buffs as much as possible. (See diagram below!)
I will try to keep the guide updated as i find new things about this boss. As said, I have only attempted him once, and this is what I noticed during this attempt. If you would like to add anything to the guide, please post it so that together we may build a great guide for such a difficult boss!
Thanks to Luckie from Siel for contributing the information in method 2!