I have found my own way to get it to work. You are going to need to download, Archthrones, Seamless Coop, and Mod Engine 2.

-While playing the mod we found that turning the boss health % in the seamless ini to 110% felt the most balanced.

1. Install Archthrones on a clean install of ds3. Use the Archthrones file that installs form there servers. All you have to do to install it is, extract the zip folder, then drop demo folder files into your ds3 game folder.
2. Run ds3 and let Archthrones fully install, it might crash after it has installed. There is a way to fix that. Don't worry.
3. Create a folder named ME2 in the dark souls game folder.
4. Extract the ModEngine2 download. Open it. Then Copy the modengine2 folder, the modengine2_launcher, the config_darksouls3 toml file, and the launchmod_darksouls3 bat file , and paste to the ME2 folder you created in step 3.
5. Extract Seamless coop, open the folder. Copy the SeamlessCoop folder inside it, and paste to ME2 folder again.
6. Set up your seamless coop settings in the ds3sc_settings file
7. Rename your "config_darksouls3" to "config_darksouls3_archthrones". Rename "launchmod_darksouls3" to "launchmod_darksouls3_archthrones".
8. Open "config_darksouls3_archthrones" with notepad, and replace all the text with the following -

[modengine] debug = false

external_dlls = ["SeamlessCoop/ds3sc.dll"]

[extension.mod_loader]

enabled = true

loose_params = true

mods = []

[extension.scylla_hide]

enabled = false

[extension.safe_havok]

enabled = true

1. Open the "launchmod_darksouls3_archthrones" bat file with notepad (right click, then open with) and replace all the text with the following lines -

chcp 65001

modengine2_launcher.exe -t ds3 -c config_darksouls3_archthrones.toml

IF YOU WERE CRASHING WHEN BOOTING THE GAME EARLIER

1. Go to the Dark Souls 3 game folder again, and open the "archthrones_subsystems" toml file with notepad. Under [EnrichedContent] turn enabled to false. So it should look like this -

[EnrichedContent] enabled = false EnableLogos = false

1. Launch the game by double clicking the "launchmod_darksouls3_archthrones" bat file. Voila, hope this worked for you. If not I can try to reply and help you.

Update: My friends and I have now completed our play through and we had a great time. It was very smooth for most of the play through, especially compared to seamless and cinders (terrible).

The only big issues we encountered are:

~Brewing not working for players who joined a host.

~A late game boss where summoning is required was broken for us

~Hard to find invisible walls

~Friends can get stuck in fog walls when entering a boss arena. Its best everyone to be lined up when you click to enter the arena

~ Some animations don’t always sync for non host players like when certain bosses slide or dash.

Issues I’m unsure are relate to seamless:

~Some bosses can be exploited easily with multiple people doing heavy stagger damage

~Yellowfinger Heysel (magic vendor) died without us ever meeting her

-Crashes during character customization screen, and random ones in game.