SWORD ART ONLINE: FATAL BULLET

Dissonance of the Nexus DLC

This boss is story-related and can't be missed. It will basically be a repeat of the Freezing Menace, only it will do nearly twice as much damage and have twice as much health.

You can beat him 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.

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):

Personal Skills:
-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

Gadgets:
-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)

Friends:
-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)

Cheva1ier's solution is pretty much in-depth regarding the boss patterns. However, I wasn't sure on what sort of build to run, after struggling with my own build I used to get through all of Extreme until Nifta v2 and the souped-up Ice Queen (who is argubly far more difficult in comparison to Nifta).

While this is not really a solution, I was able to make a build that has, to put it blunty, able to cheese some of the battle. Not entirely, you'll still need to put the work in to get through it, but you get a bit more leeway with the "last wind" style here.

Before we get into this, make sure to save your current build in one of the customization slots at your home terminal, because you will be respeccing to get this to work. This is based on a Level 300 allocation, which I recommend grinding to for an easier time in general.

STATS:
Basically, this build will allow you to get the required skills, while also boosting your crit damage. Crit damage is important for the DLC bosses in general.

Next up, I wanted to focus on using an AR, it's a personal preference, of course, but having a good AR is always a bonus anyway.

I used the Sylph Type-D Legendary (Rank 11), which drops from the giant stinger above the Ice Prison entrance. Having all of Argo's cologne intel active will give you a generally better roll while farming, but you may want to stock up on multiple copies. After you're satisfied with your rolls (providing you have at least four of the forementioned chips), feel free to buff up your gun with the following chips.

WEAPON CHIPS:
The amount of upgrades you can perform on a chip depends entirely on Lizbeth's Workshop level. If you didn't go to NG+ on Extreme, you're going to be somewhat handicapped, but it's still defiantly possible to make this weapon shine as long as you minmax these chips using the Predatory Shop (again, on Extreme) and rolling for what you need there if you're not sure about commiting to farming weapon rolls from bosses.

For skills, I only had to use four (you can use up to eight with a second weapon).

SKILL SET:
Human Fortress generally works like Armor Form, but it includes the benefit of knockback immunity. This is absolutely amazing during boss fights as you can get out of a rough situation without being left open for the next hit. Likewise, Shield Matrix allows you to cheese through ANY attack. This is great, because you can laugh at Nifta's dumb laser sweep, and also use it as a denial for the spin attack if you're unable to set enough distance apart.

For accessories, put on anything that boosts DEX and/or LUC, but also be sure to throw on Avoid Instant Death, as it gives you a safety net from one hit kills when at full HP. The effect only seems to trigger once during your current life (it'll reset after being revived), so don't rely on it too much.

ArFA-sys is a bit of a different story. Making them a bit more tanky helps as they'll be able to sustain some of the heavy hits, but unfortunately will fall to the one-hit shots.

You can build them however you'd like since they're mainly support here. I set the following so they could dual wield the HG Kuroneko. Is that Revolver Ocelot? No, sadly.

STATS:
SKILL SET:
For accessories, anything with Defense vs. Mechs and Avoid Instant Death will suffice. As long as they're not running around the battlefield like a drunk while you're incapacitated, you should be fine. Hopefully.

Lastly, the gadgets you should use are First Aid Kit 2, Anti-Aliment Kit 2, and Resuscitation Kit. Fill the last spot with anything you want.

Also, it should be common sense at this point, but never huddle around your AI party. Keep your distance from your friends and foes.