So gonna post the mechanics and strategy I've used in my solo ambu video which hopefully helps lower populated servers beat it and help people low man if they don't have many people available.
In the video I only showcased Normal since I believe the majority of people will be able to do that low man much more than D. Imo, low manning D is considerably harder so I don't think it'll be worthwhile showing unless for bragging rights, idk. Sorry about long post, I just copy/pasted description.
Mechanics:
6 mobs, 3 dark affinity, 3 light affinity. The mobs that cast Wind, Fire and Thunder absorb light skillchains and the element that cast. Polar opposite with Dark affinity. They absorb darkness and the spells they cast.
The more of 1 type of "mode" you kill, the stronger that particular "mode" gets. In this video I killed only Warrior mode. The reason for this is because /nin Utsusemi makes the Warrior modes almost useless. However, if you notice in the video, I only silence the "Mage" mode AFTER Manafont has been used, that is why not a single Mijin Gakur goes off.
The mobs have a "Resist Sleep" job trait which is why Sleep sometimes gets resisted. Same thing happened with Silence, however, not nearly as often and obvious. This means that no matter how high your magic accuracy is, you can still get resisted on sleep, even with Elemental Seal and Stymie.
Strategy:
Buff up as normal, AAHM, Amchuchu, Yoran-Oran, Ulmia, Cherukiki trusts.
give all trusts Shell V for the initial pull. surviving the initial pull is the hardest thing about soloing this ambuscade to be honest. Trusts dying isn't the end, just makes it faster.
Saboteur + Spontaneity sleep 2/1 rotation on each mob and try Addle each mob you fight. I tried to be lazy on debuffs to show it's not the end of the world if you don't debuff them, but I wouldn't recommend it.
Any mobs that cast Fire, Thunder and Aero I try avoid light skillchains, and if the mobs cast Blizzard, Stone and Water, I then tried to make light as often as possible.