Dissonance of the Nexus DLC
This boss is story-related and can't be missed. You will only need to beat it on Normal, but the achievement will also unlock if you beat it on Extreme.
This boss has minor weak spots in its knee joints on its legs, but its maximum DPS weak spot is its face, which is a narrow orange/yellow slit on the front of its head. So you will generally only be able to hit it if it is looking in your direction after one of its spin attacks.
You will want to stay as far away from this boss on the other side of the area. This is due to his spin attacks, armor/power debuff freeze cloud, and his near-death mode when he shoots homing icicles. Being far away gives you time to run to one side and dodge roll out of the way.
During the near-death part, after the homing icicles leave giant icicles around the area, you will hide behind these icicles to avoid his slow spinning laser attack that does massive damage.
If you can stun the boss or one of your friends does with a skill, try to stand in front of him as he will simply lean backwards while stunned. This will be your only static chance to shoot him in the face a bunch of times.
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 (level 275):
-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)