My first ever steam guide so I don't exactly know what Im doing here.
Hope it helps though!
also let me know if there is something i should update
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3243147778
Please refer to this in the case you want more information about combat

This door and its contents are what I am referring to in this guide, if you have any other questions relating to it or another boss Im down to try and help, but this is my main focus. https://gtm.steamproxy.vip/ugc/2469734902815186713/25E05F4DE6E4712C616C2AD6C4E9D5B45F32DECC/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

From here on is extreme spoiler territory you have been warned

As soon as you enter you will be greeted by some platforming, it will be pretty challenging
Basics
per character you have your 2 movement options.

Blue has dash and sprint.
Blue's dash is the default binding of RB/R1/D and propels you straight in a straight line along the X-axis, or up along the Y-axis. If you dash into a wall, you will land on it in a grab pose, while in this animation you can use sprint.
Blue's sprint let's you run fast along the ground, and along walls. you will do a flip off of walls the you sprint over.
Orange has grapple and double jump.

Orange's grapple will only activate within the circle surround an orange grapple point suspended in mid-air and throw you out the opposite end of the circle after activating.
The double jump is only available while orange is on the screen, so you need to swap to them if you want the extra height.
Both Dash and Double Jump get refreshed after landing on a wall or using a grapple point.




IF YOU ARE READING THIS BEFORE COMPLETING IT PLEASE DO SO BEFORE CONTINUING
Following this, there is a checkpoint that allows you to come and go freely, but you cannot yet replay the path of determination. Don't worry though, once you beat this next portion you can retry at your leisure.

I hope you have actually seen this part before reading this.
if you have welcome back.
if not... oh well.

THIS is the hard part of the south branch. The boss.

VERY IMPORTANT TIP
using orange's magic ability SEAL will reveal most shield affinities that are active.
From here on called Summum, is no joke. This boss took me a good 5 hours of head banging to finally get. Which happens to be the sole purpose of posting this, because I don't think its very fun to stay at the same point for hours on end with no improvement.

Boss fight overview.
this boss has 4 phases, all of which are challenging in their own regard

phase one
the boss will start with its default shielded state.
Not too difficult, just get used to the different attack patterns
has 5 shields

phase two
The currently active character after the boss recovers from the shield break stagger will become a puppet, fighting you with basic combos and using any ability charges they have. Following the defeat of your other half, they will return and Summum will go back to phase one behavior.

Phase three
The start indication occurs when the shields regenerate, or after a certain amount of time passes.
After the set period of time, that boss starts glowing brightly and grows what looks vaguely like a halo.
2 new mechanics and new attacks appear exclusively in this phase.
mechanics
1. The halo will frequently pulsate throughout the duration of this phase, reducing progress on all active shield types.
2. Two floating stars will one at a time hover above the player and attempt to fall on them. To dodge these, you must either advance toward Summum, or use an attack with evasive properties.

Attacks
1. Many of the attack patterns from phase one will become fusion attacks.(star attacks that you must dodge or parry).
2. A new attack which is a circle that encloses the player and tries to collapse on them. This attack is magic based so it can be absorbed by orange's defensive special. otherwise you must dodge or parry(i don't know if parry works on this as i've never tried)

DESPERATION PHASE
this phase will likely be the cause of some trouble the first dozen tries. To get here consistently, I used orange and their
Summum, after a set amount of turns or once recovering from a stagger will slam the floor and start charging a Mega flare. This explosion will deal A LOT of damage, equating to roughly 40 ish percent of max upgraded health. there is a secret achievement to be unlocked by beating Summum before this attack even shows up

This explosion occurs 2 offense phases after the animation starts. with the falling stars attacking you on the second phase.

This attack cannot be dodged.
however, it can be counteracted by waiting out the halo damage phase and only breaking the shield once the desperation starts, this staggers Summum and brings it back to phase 1 behaviors as far as i am aware. But this strategy provides significantly more risk as you take so many more dangerous attacks that it is hard to manage your health.
If you survive this attack chances are that you will go down to one shot, prior to this attack starting, i recommend saving one of orange's defensive specials to heal back your health as to not screw up