Tango Fiesta

Either the achievement description is wrong, or I am misinterpreting the definition of New Game +.

The achievement unlocks when you beat Story Mode for the FIRST time. So technically, this achievement unlocks when you START new game +.

Not that I am complaining anyways.

On a side note, I found the final boss fight to be extremely difficult. Not so much the boss itself, but the floating enemies that fire explosive orbs that take out about half your life bar per hit.

I was actually able to get lucky during this fight. I started a 4 player game and had the three dummy players (I was running this solo) throw all their grenades at the very start of the match (trying to kill off those annoying floating enemies).
Anyways, 3 of my guys died quickly and oddly enough, all enemies (including the boss) began to focus all their attacks on the group of dead allies. I was able to use my last controller to literally walk around the level tracking down the boss and taking pot shots at him while he completely ignored me. As soon as the level ended the achievement popped.

This achievement is difficult since there's a massive difficulty spike starting in World 5-2 with the introduction of the flying enemies with the exploding projectiles, along with the later introduction of the soldiers shooting homing missiles in World 5-3.

I actually got extremely lucky. I was very much stuck for a long time on World 5-3 with the three objectives, having to blow up one mainframe and shoot down two attack helicopters. I could get close to beating it, clearing two of three objectives by staying constantly on the move from the flying enemies, but I would always ultimately succumb.

However, on one occasion, a glitch occurred while retrying World 5-3 where only the one destructible mainframe objective loaded and neither of the choppers did. I ran directly to the mainframe, threw two grenades at it, and the level ended.

The final boss battle in World 5-4 against the Arbyter is a nightmare on so many levels.

The only way to possibly survive this fight against the 3-4 constantly respawning flying enemies that kill you in 3 hits is to have two controllers, load in both players, play as Player 2 and just let Player 1 die.

The Arbyter will only target Player 1, even after they have died, with the exception of his homing missiles and his pink slime.

Even so, the flying, exploding enemies will still target you as Player 2. I did not find a reliable way to escape them at the level start and had to die and restart repeatedly. It's also hard to escape them off the level start, as your screen will not move until Player 1 has died.

Since the levels are procedurally generated, some layouts will be more favourable than others to winning. Whenever possible, I would try to flee the starting area and just ignore the Arbyter completely initially while trying to run away from the flying enemies so I could hide behind a corner or an obstacle so the 3-4 flying enemies would target Player 1's dead body or glitch and get stuck on a corner.

Sometimes I could get away from the initial drop area with only one flying enemy chasing me, so I could eventually kill it and remain far enough away from the main fight so it wouldn't respawn.

I would recommend equipping the Cobra Cleaner in one of your two weapon slots. I tried this level with many weapon combinations such as the Arius A1, Woodstock 69, the Viper Combat rifle, Muzzle Tight Puppy etc. I found I had several runs where I was able to escape the initial fray and hide on the far side of the map to take potshots at the Arbyter, only not to do enough damage and to run out of all ammo and grenades when the Arbyter still had at least 25% of his health left, forcing myself to suicide and have to start over.

I also noticed that the human enemies that provide medkits and ammo at the start of the level seem to stop respawning early on and the flying enemies have bubbles indicating rewards, but never actually drop anything upon death.

The Cobra Cleaner has the advantage of taking off huge chunks of damage every shot off the Arbyter's health bar, while coming standard with over 300 rounds of ammo, which should be plenty.

It also seems that only attacking the Arbyter's arms and legs actually does damage to the boss, at least until the final form. So don't bother attacking the head or the guns on the torso or the torso itself. You will have to choose a limb to attack, follow the Arbyter around as he repositions, choose your attack angles carefully and conserve ammo as much as possible.

I would also recommend avoiding the use of the Matrix 80 or the Viper Assault rifle. I read a review of the game which suggested some equipped unlockable weapons make the game unstable, and this fits with my experience. The game crashed on me when I was trying to unlock the Matrix 80 achievement and temporarily corrupted my profile. Similarly, I found the use of the Matrix 80 make the game's performance during the final boss battle very unstable, crashing often.

Even without equipping these weapons, the game's performance on the final level was terrible. The game would sometimes freeze for 5-10 seconds at a time, only to eventually resume. On one run where I had the Arbyter down to 1/3 health, my character froze and became unresponsive whereas the audio played and enemies continued to move as normal, forcing me to restart.

I've included a few videos (not mine, credit to YouTuber FelipePsyco35) of someone successfully beating the final two levels