TL:DR Play the world on Easy, I advise going a culture/society that is heavily geared towards industry/knowledge and going down the Chaos affinity tree. Turtle and settle away from the Warden, go for a magic victory. Scout heavily early game and set up a network of scouts/watch tower outposts between you and the warden. Ignore your allies, unless they're being killed by the warden.
There are too many players on the map to win an expansion victory and your primary enemy is very far ahead of you for most of the game so a Magic Victory would be the optimal win. The Chaos playstyle is for two things: 1) There is an unlock near the end of the tree that gives gold and magic income for each war you are involved in, the scenario forces you into war with many players, providing you hundreds of gold and mana income per turn. 2) Chaos can rapidly summon high tier units to replace your losses from fighting the strong armies that come to pillage your lands. My armies were entirely made up of Ironclads (Warbreeds? can't remember) and Tier IV Fiends that could be replaced in 1-2 turns per.
At the start of the game you have an option between 4 game-long boons for you and your allies. I chose Lithy'ls ritual which can freespawn Astral creatures from unit deaths on the world map. It gave me many scouts early game and many Tier IV Mimics mid/late game, but from how many battles I fought I think I would've prefered Yaka's ritual. If you plan on turtling entirely I'd take Ydgaard's ritual.
You have a good bit of time before the Warden starts bullying you and none of the other enemy players can truly threaten your allies, your job is just to keep the Warden on you and stay alive until you can get a Magic Victory. Scout as much as you can, find the Warden, settle away from the warden and plant some Watch Tower outposts and scouts between you and him. From then on, play the game as normal and you should scrape by, if not get an easy win. I finished this scenario while also doing the Master of Evilness secret achievement since it requires going down the chaos tree anyways as far as I know. Play as a Dark Goblin Champion and do a Chaos Magic Victory. Good luck!