Sunset Overdrive

This achievement is surprisingly luck based based on (where you join the game, what levels you are given, who is in your lobby, What chaos you are given, who your teammates are, how many people follow through to the end) and that is all before you even get to the 'defend the vats' part. If you get to the end part with lower then 1625 chaos then you can't get the achievement at all and you have to do the mini missions all again (You will be told how much chaos you have just before the end defend mission). When you get there however here are some useful tips:

Unlike the other three chaos missions in my experience it includes all of the special OD

It seems that only the normal OD can wear down the vats so focus on them

Special OD break traps so replace them when they are destroyed

For how I did it, I essentially just spammed every explosive weapon and pyro trap I had into the OD spawning locations as you can shoot through the invisible walls on the sides. This works until wave 3 where there is so many special OD I just alternated between being dead and spamming weapons until the game ended. For the special OD in the air I just ignored them and I managed to complete it.

For what I would suggest, Use the freeze gun as it gives you time to breathe and use explosive weapons such as the teddy bear launcher and the hair spray launcher as they have large blast radius' taking out many OD and they damage the Special OD at the same time.

Alright, AN ANGRY HOBO i has a lot of great tips for the Night Defense but I'd thought I'd throw in my two cents.

As AN ANGRY HOBO i pointed out, only the normal OD actually take overcharge away so focus on them! If you can get Blowers in the process so be it, but trying to kill Wingers and Herkers is just wasting time and manpower from the normal OD. Just try to avoid the Special OD as best you can.

When I finally completed this I had 650+ kills, the next person under me didn't even hit 200. Even then we still lost quite a few Overcharges and barely squeaked by. If you want a fighting chance to win this, you'll most likely need a couple of people in your chaos squad who can blast the living shit out of the normal OD.

This is the setup I had and it certainly packs a wallop:

Weapons (All Level 5)

Hairspray Bomb with Second Amendment (This is your go-to weapon. Use it as often as possible and only switch to TNTeddy if its ammo runs out)
TNTeddy with Twist of Fate

These are optional but highly recommended:
Acid Sprinkler - Good for putting by barricades when hordes of OD are coming
Freeze Bomb - Nice for when a vat is being overrun and can temporarily stop OD from taking anymore Overcharge
Shorty Shotty - If you run out of ammo for others, this is a decent gun to blast many OD at a time until you get ammo back for Hairspray and TNTeddy.

Overdrives (All Level 4)

OD Killer
OD Rival
Sacrifice - Weapons
Ammo Capacity - Single Shot
Sacrifice - Single-Shot Weapons
Weapon Damage - Single-Shot

^^^ All 6 of these are extremely important (as Hair Spray Bomb and TNTeddy are both single shot and the backbone of this setup) and wouldn't recommend switching any of them out.

Amps

Melee - Twister Spin
Hero- Sadomasochist
Epic- Chance of Lightning
Dive Bomb- Seismic Smash
Hero Extra- Air Power

^^^ These are the least important and you could probably switch these with Amps that are more your play style. I would at least recommend Sadomasochist because near the end, you'll never have much health and the added damage will certainly help.

With this setup I was often able to totally obliterate hordes of OD before they were able to break through barricades even when it gets insanely chaotic near the end. Also, once Wingers and Herkers spawn I wouldn't worry about Traps (Those special OD can wipe them out instantly). They just end up dividing your attention from other important matters.

Hope this helps to anyone having problems with this (I know I did) and good luck not getting noobies in your squad!

1) The first part of the job consists in getting the said 1625% Chaos from the sum of the minigames BEFORE the final battle (the Defend the Vats last stand doesn't add more Chaos to the count).

2) In the defend the vats final battle it's gonna be chaotic. So, remember that if there was competition between teammates it should have stopped at the last minigame. Now you and your team needs to work togetha and do a great job at it. That means spreading out instead of everybody trying to focus on a single wave of enemies and also focusing on the exclamation marks if they ever appear, which means that the enemies have breached your defenses. Try to ignore the Wingers in the air and do whatever possible to stop the big guys on the ground, but not before stopping the regular enemies. They are the ones who count.

I would also add that in order to make this achievement "easier" and possible, it'd be good to have at least 1 or 2 skillful players in your team. People who can carry the extra weight, both in order to get the Chaos points up to the required percentage and also to make the difference in the last battle.

Last but not least, here's the minigames I recommend picking and the ones I recommend avoiding when voting:

Wrecking balls - this is a good minigame to give everybody a chance to earn the bonus multipliers in the post game spin screen. Nobody is competing with nobody, except themselves. One bonus objective is to kill 20 enemies, the other to kill 40 of them.

Vending Vandalism - a nice game but one that requires excellent teamwork in order to complete it under 2 minutes. The first set of 9 machines spawn right where you are and everybody will probably take part in their destruction. As soon as the "Go to the next area" warning appears, instead of going towards it, if you're already used to this minigame and map, go to the third spot instead (it won't be marked on your screen so you need to know how to get there, which is easy, pretty much just follow the road hugging the invisible wall). Then, right after your teammates finish cleaning up the second one, the final 9 soda machines will appear and you can start destroying them, without awaiting for the rest of your squad. That's how you complete it efficiently in under 2 minutes.

Day of the OD - of all minigames, this is the one that I haven't played many times at all. So, no good tips here.

Fizzco Ambush - THE BEST mission in my opinion. Should be selected every time it appears in the voting screen. As soon as the game starts, pull out your Hair Spray Bomb (preferably equipped with the Second Amendment amp), jump forward and move down the road shooting everything until you don't see any red dots on your radar anymore. Keep bouncing on cars for extra points. Then start going back for that's where the next robot enemies will appear. Again, keep jumping on a car. When you get good on this mission, using the mentioned weapons, you'll get the most kills in it everytime. All in all, it's a short mission with lots of explosions and quick points.

Defend the Hospital - My tactic here is to stay away from your teammates from the beggining. As soon as the game starts, go South (check map) towards that big blueish building that looks like an underbridge. Stop between the numbers 4350 (on the right) and 4020 (on your back, on the left). From where you are, enemies will spawn in a few seconds, coming from 3 different doors. You can set up Acid Sprinklers and Turret Copters, besides shooting with your Hair Spray gun. Keep bouncing on the top of a car for extra points. It's guarantee that the biggest score and number of kills at the end of the round will be yours.

The Floor Is Lava - 1st golden hint: instead of rushing to the start point where everbody spawns, stay BY the Vat above the lava part (coming from the starting point of the mission, the first Vat to the right). That way when the mission starts, you'll be ahead of the others and really close to the first "pushie" doll. 2nd: after delivering the first doll, go back using the same upper way that you came from instead of jumping down and trying to slide through the lava like most people do. Keep doing that until you've collected as many as you can (I've collected all 6 a few times that way). Also, pay attention to your radar, it will show the locations of the dolls. Sometimes a couple of them spawn in a very safe area, away from the lava and close to the delivering point.

Scrambler... something - NEVER choose these "collect the numbers" kinda mission. They are one of the easiest to fail, no matter how hard or excellent one team member performs.

Fizzco HQ (a.k.a. "destroy the boxes") - In time I started hating this mission, for I was divided between finding and destroying 10 jedi robots for the bonus objective or focusing on destroying as many boxes as I could. Truth be told, it's hard to find more than 5 blue sword robots, left alone be able to kill them undisputed. Depending on the other option at the voting screen, I'd pass on this minigame.

Urban Jungle Gym IV - The least generous minigame to choose from. Only 1 player will get both bonuses for its objectives. After I grew a conscience, I stopped picking this game when I had a better option to vote.

If the tips mentioned on this solution help you in any way, please give it a thumbs up to let me know. Thanks!

Doing Chaos Squad with 2 people.

Nobody is playing this game now, and I am too lazy to go find a group. Me and a friend decided to tackle all Chaos Squad as two people and we have been successful. You have to set your Chaos Squad to private, invite the second person and then search. After a few moments it will put just you two into a game.

With only two people you have to be at the top of your game. You both will need all of your guns at level 5, and your other amps as maxed as you can make them.

The Guns we used were:
Hairspray Bomb w/ Second Amendment
Acid Sprinkler w/ Winter is Coming
Taunt Bot
Pulse Mine
Rager (Rager is better than Plague Bomb because Rage stops them from attacking the Vat. Plague does not.)
Swarming Nanobots
Freeze Bomb
Shield Buddy

You can use whatever amps you feel like, I never noticed too big of a difference except with freeze in the Acid Sprinkler. That is a must and will save your life.

For the personal Overdrives I would use OD Killer, OD Rival, OD Less Damage, Health upgrade, Deployable Ammo, and Single Shot Ammo. Damage on your guns is not terribly important because things are going to die anyway, you need sustained survivability and enough ammo to get through.

Always vote for the highest Chaos Squad in each mission. Generally if you vote that way you will reach the Chaos Squad goal. No matter what avoid Scavenger Scramble. The points one is unwinnable with two people. You really want the game to give you more Vat Capacity but you don't really have a lot of control over that.

The Night Defense has 5 waves. The first 4 waves have set waves of enemies. Once you defeat all the enemies the wave ends. The 5th wave is times. There is still a set number of enemies but it does not end until the timer is up. Even if you kill all the OD you can still lose if you get killed by Wingers. Speaking of Wingers, ignore them. Wingers, Herkers, and any other special OD. They will not attack the vat, so they do not matter. They will attack walls and traps, but the only thing that matters is killing the normal OD. This does make things difficult in later waves when there are a ton of Wingers around ruining your ability to grind, but you have to adapt and endure. With two people you can't focus on killing wingers and OD, and you gotta kill OD.

For each Night Defense I would put an Acid Sprinkler, Taunt Bot, and Pulse Mine in front of the gate. Then I would use Hairspray Bomb to attack. In the later waves when there is just too much going on I would focus on whats around the vat using Rage on large groups of OD. Acid Sprinklers around the Vat is a life saver.

For Traps I generally only used Senior Freeze. I would put them around the vat or in the path of the OD if the OD always followed a set path. Once there are a ton of Herkers and Wingers the traps get destroyed too fast. I would just put down a machine gun turret to help.

Downtown Specific

All of the missions except Scavenger Scramble are doable with 2 people. You just need to pick the ones with the highest Chaos. Togthawiistand has a very good guide explaining how to complete each mission so I don't need to explain anything here.

Night Defense.

This Defense is very very hard with two people, but not unwinnable.

First we gotta get the lay of the land. There are 3 vats. Two are in the "front" with one vat in the "back". This description will assume you are facing the direction of the two vats with the one vat behind you. The two front vats will be known as left vat and right vat. There is one gate in the front by the two vats. There is one gate on the left side and one gate on the right side next to the left and right vats. In the back there is one gate next to the back vat and one gate above the vat.

Depending on how much trap energy you get the set up I would normally do is place Senior Freezes in front of the gates in the middle walk way between the two front vats. I would put a machine gun turret by the back vat to shoot anyone that gets through. If I had more I would put the senior freezes as close to the vats as possible. Unfortunately there is not enough room for a senior freeze next to the back vat.

To beat this Night Defense with 2 people you have to stay in constant communication. The secret that makes this winnable is that the OD will only attack from two Gates per wave. What gates they attack from are random, but it's only two.

The way it works is that at the start of a wave a large OD force will attack one of the gates. Once the large force starts attacking a second group of stragglers will attack a second gate, generally on the opposite end of where the first attack is. Because you do not know which gate is going to be attacked you have to grind around the level until you see the force and then drop all your Deployables. Once you get things under control one of you must go looking for the stragglers. You can't both go looking because one person has to fight the first group.

Thankfully once the OD have picked a gate they won't attack from any other gate that wave. When the wave is over you need to go back to looking around because they will pick a different gate. They can pick the same gate. I once had a run where large OD group only ever came from the front gate.

Eventually there will be a ton of Wingers and Herkers. Like normal you have to ignore them and just focus on the OD. It is really easy to die but you pop right up in the middle of the area so it's not hard to get back. Herkers will break down walls so you have to be ready with Freeze Bombs and Rage to control the groups.

This is the second hardest Night Defense to do with two people, but it is very doable.