THERE IS NOW A PERMANENT SAFI'JIIVA MOD WHO HANDLE MULTIPLAYER SCALING:
Get it here: https://www.nexusmods.com/monsterhunterworld/mods/3197?tab=description
Mod made by HGDF7C213GF7D8F 

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Replace Safi'jiiva Siege quest with a version for solo player:

Changes:
-Monster's Health is divided by about 2.55 (this is usually by how much solo monsters' hp get scaled up for their 3-4 players versions).
-Timer is increased up to 50 minutes
-Part's Health is reduced (might be too low but it should allow for the player to break everything)
-Status and stuns resistance is lowered (except to mount)
-This quest is limited to one player

Attack, defense and quest reward stay the same. 

There is already a mod that aims to scale down safi'jiiva by NekotagaYuhatora: https://www.nexusmods.com/monsterhunterworld/mods/2909
This mod actually handles proper multiplayer scaling (meaning safi'jiiva will get more or less hp depending on the current number of hunter participating in the quest). However the hp aren't properly scaled down and the time limit remains 20 minutes. What I really wanted was a safi'jiiva quest that would be like a normal hunt, without having to redo it multiple time to lower the energy levels. If my version of Safi is too easy then I encourage you to try NekotagaYuhatora's mod instead.  

Obviously requiert Stracker's Loader.

HOW TO INSTALL:
-Install Stracker's Loader
-Copy paste the nativePC folder into MHW root directory (the one with the MonsterHunterWorld.exe file