And I'm playing on casual, for the record.

His first phase is not a problem.

It's the second when his head starts to float and all the walls go down, revealing an insane amount of monsters from every direction.

It's weird, because all the videos I've watched on YouTube of the fight with this boss, don't contain these monsters at all! Just the floating head!

I'm guessing that it could have been introduced in the latest update, and the YouTube videos are of the initial release.

As of now I can't continue the game.

Is anybody else struggling with the World 3 boss on the latest update? Those of you who did defeat him, how?

Unfortunately that doesn't work.

I'm running, and I've been using a combination of toughness, sprinter and ghost perks. I've been trying to quickly destroy these monsters with dynamite or upgraded minigun, but the dynamite is very ineffective and the minigun ammo runs out too fast even with full magazine.

There are lots of different projectile shooting monsters, some of which you have no way to avoid, and there's nowhere to hide. It's impossible to clean them out before they kill me.

It just can't be done.

I spent +30 lives (and I had to start the game from the beginning) on normal, but once I realised I can run it was piece of cake.

Just keep moving and strafing, it will be easy

First phase
-Stay where you are, behind that wall, don't advance or explore
-Just keep shooting the cat from the left corner
-When you know is about to die (I didn't activate the health bars for the enemies), go to the right and kill all the monsters that spawn in front of you (so you will have an easier time when the walls disapear)
-Finish the cat

Second phase
-Well, just run and don't stop running. Kill most of the monsters but focus on the cat, since monsters continuosly spawn is a waste of ammo tring to get rid of the monsters
-Once the cat is dead finish the remaining monsters and that's all

That doesn't work either.

I'm still getting wrecked every time by his monsters before I even kill the head.

They're everywhere, so it's not like I can avoid touching them even if I have the ghost perk, and some of them hurt me with rays that can't be avoided. I am running and moving but it doesn't matter. I can maybe get the floating head's HP down to about one third before they all kill me.

I have 220 HP with toughness perk, and also ghost or sprinter perks, I'm using upgraded minigun, but they still kill me.

Unless there's a better weapon I can wreck them all in like 5 seconds tops, I don't see how this is possible on casual for anyone.

Ok so I decided to give this a try again now that the game is patched to be harder. I died on my first try when I decided to try to concentrate on the adds. On second try I just dps the boss and circle around so I don't get hit by anything, basically same tactic that worked on release version, just takes longer time. My stats were 2,15,8,5; perks toughness, student, quick healer. Weapons I used upgraded staff and minigun, non upgraded double barreled shotty. Good luck to you! (I'm kinda intrigued to test upgraded flamethrower on adds for lols)

It is a considerable difficulty spike, but it is possible with the right tactic (played on normal).
First - there currently is a bug with moving speed. At any point deactivate and reactivate running - you will be moving faster then.
Now, the important part - the monsters spawn only in the first stage, when the Sphinx is stationary - the sooner you take it down, the less monsters you will have to fight. So ignore the monsters as much as you can until the head takes off.
At that point, when the walls go down - ignore the head and focus on the monsters. If you keep circle-strafing at a biggest possible radius they won't be even able to hit you. You can actually take your time at killing them then (but you should kill the summoners first). The flamethrower is extremely effective. A single flak cannon bullet will also kill most of the enemies in this stage. Once that is done finish the cat off.
For me the first stage was much harder, because there of the limited space.

I'm sorry but this is simply not true.

There are these pink demons who shoot rays at you which are always instant hit. No matter how fast I'm running. The damage they do is considerable, even at casual difficulty, either their damage needs to be nerfed, or they need to be removed altogether from this fight. Otherwise this boss will remain unbeatable.

So I just tested it. My mistake - those are the only things that can hit you. The damage seems fairly small though, but that depends on how many enemies there are.
However - new strategy: just throw dynamite at that big cat. He won't have the time to summon anything - tested on hardcore. The first form was down with no enemies summoned.

It's not the issue of him summoning these monsters though.

The problem is that as soon as his first form dies, all of the walls go down revealing a shitload of monsters, including ray shooting pink demons. They come at me from every side, and I can try:

running around in circles and shooting the monsters while the monsters and head kills me.

or

running around in circles and shooting the head while the monsters and the head kills me.

Either way I'm screwed before I manage to kill one of the parties.

There really is no way to win this fight.

It's weird, because all of the youtube videos I've seen of this fight don't have these wall monsters, but I assume they must have been added in one of the updates, while the videos are probably from the initial release. Probably the developer of this game wanted to make the game more challenging, but instead made it unbeatable.

That's what I tried to say earlier - they won't be there if you take down the first form fast enough.
They are being summoned as long as the first form is alive. The longer the first part of the battle takes - the more monsters you will have to deal with. Kill first form really fast and there won't be any when the head starts flying.
I've tested it on hardcore just a few minutes ago.
I'm on version 1.0.0.2.7 (I'm not using Galaxy and there was nothing really important in the latest hotfix).

I can record a video if you want.

It's super easy with high strength an upgraded Axe and the strong third strike perk. I just got close and hit fire until the Sphinx is dead. I remember that I went through the first phase so fast that no monster spawned (normal difficulty).

Hmm. I didn't have a problem with this boss, and I just beat it on the most recent patch. My stats are 2, 5, 22, 3, with Treasure Hunter, Student, Magic Potential, and Soul Collector.

I focused the head right from the start with Lightning Staff, took it down while weaving in and out of the terrain/columns, then when the walls dropped I just planted a Storm Rage and everything disappeared right quick.

As other people pointed out, the best strategy seems to be to focus the Phase 1 of the Sphinx very fast.

I have no idea how I'm supposed to kill this thing. I just don't have any weapons that do enough damage to kill it and it's hordes of minions in the hand full of seconds I have once the walls come down. My stats are..

Strength - 5
Life - 8
Spirit - 1
Capacity - 10

I chose Quick Healer, Treasure Hunter, Lead Collector and Toughness for my perks. I also chose to upgrade my explosives to proximity bombs. Which are super shitty. I upgraded my axe with the healing ability, which sounds super useful in the description but the healing is so tiny it is effectively useless. I haven't purchased any spells, because the weapons upgrades always seemed more useful.

I can survive just fine before the walls come down. The time between when the walls come down and when I die is just a hand full of seconds. I'm not sure if my choices ( which are irreversible ) have inadvertently screwed me or if this boss is just impossible to fight. Ether way I've been looking for some kind of strategy or cheat to get past this one cheap ass boss.

I've tried some of the thing so fare suggested in this thread. The fastest way to kill the first stage is the mini-gun... and that's not very fast. I've tried using bombs to kill it faster, I've tried to kill it with my healing axe, both lead me to just get killed before the second stage. I've tried using my flair gun or shot gun to kill the head, took forever and left a massive hoard of monsters.

This is really ruining the game for me.