Dissonance of the Nexus DLC
This boss is story-related and can't be missed. It will basically be a repeat of the Snowstorm Queen, only it will do nearly twice as much damage and have twice as much health.
You can beat her on Normal first to get an idea, and then switch to Extreme and make your attempt for the achievement. The boss is deadly on Normal, and can still kill a lvl 275 easily.
You won't need much of a strategy. One of the only subtle differences is that there are 3 orbs instead of the 2 during the Snowstorm Queen fight with the third orb hovering near the entrance. (I'm not even sure what they do, but she does have a Roar attack that stuns everyone on the battlefield, or uses up your Hyper Awareness if its on. Keeping the orbs dead might have kept her from using the Roar attack consistently)
I just kept moving left/right across in the field about half the distance from her, trying to shoot the gem in her chest. If you can do enough damage to her chest gem, you can interrupt her attacks.
Her regular attacks are to smash someone with her spear, or to summon an ice orb that shoots a laser beam. Later, when she has lost some health, her spear attacks will become triple attacks. Worst case scenario, each hit from the triple knocks out one member of your party, so hopefully whoever is left revives quickly.
She will randomly, yet rarely, do a blizzard attack. This blizzard attack will be focused around whoever she wants to kill, and will do significant damage if you or your friends are hit, so just keep moving. In fact, keep shooting her in the chest gem to try to interrupt it.
You will need to set up your party around support, healing, and reviving, because your AI friends otherwise don't help much. YOU will be the main DPS, so I'll show a build strategy.
Regardless of difficulty, these will be your "go to" skills for boss fights:
-Armor Break Shot 4 (INT 193 and DEX 95) This will dramatically increase the amount of damage you can do to an opponent
-Power Break Shot 4 (INT 193 and DEX 95) This will dramatically decrease the amount of damage your opponents deal
-Power Form 4 (STR 191 and AGI 94) This will dramatically increase the amount of damage you deal (could be anywhere from +1000 damage to doubling your damage)
-Armor Form 4 (VIT 187 and STR 93) This will dramatically decrease the amount of damage you take (could be anywhere from +1000 defense to doubling your defense)
-After those 4 you can use whatever skills you want, but I was personally backing up my team with AED Shot 1, Healing Field Shot 2, Healing Bullet 2 and Heat Detonator Shot 3 for my gatling gun
-First Aid Kit 2 (from Gadget Shop)
-Anti-Ailment Kit 2 (from Gadget Shop)
-Metamaterial Camouflage (from beating Death Gun/Kirito Mode)
-Resuscitation Kit (from near the end/finishing DOTN DLC, quest reward)
-ARFA-sys (I had my ARFA-sys practicing healing or debuffing skills for most of the game. This made them into basically an Asuna/Lisbeth hybrid) (ARFA-sys stats - STR 195, VIT 255, INT 215. AGI 222, DEX 165, LUC 30) (ARFA-sys gear - Lupus 4+ and Sound Deadener 2 with defensive % accessories) (ARFA-sys skills - AED Shot, Power Break Shot, Electrodynamic Stunner, Healing Field Shot, Power Form, Armor Form, Armor Break Shot, and Healing Bullet)
-Asuna is a straight up healer/reviver
-Lisbeth comes with a ton of buff/debuff skills at lvl 275, including Power Break Shot 4 and Electrodynamic Stunner 4, Guard Field Shot 4, Armor Break Shot 4, and HP Recovery Shot 4
With this build and these friends, I was able to defeat all of the bosses in the DLC on Extreme Mode (so long as I also kept pouring on the DPS and not letting the fight drag out too long)
As always let me know if anything appears to be wrong, or how the guide can be improved.