The two enemies accompanying the boss will counter with a whirlsand-type attack when killed. Instead of killing them, they should be incapacitated using statuses. The following work against both:
Sleep, Confu, Silence, Slow, Stop, Berserk
Slow, Stop, and Sleep are great when used together because Stop freezes the runtime of other statuses meaning you have a buffer after Stop wears off to reapply the status as the enemies will still be incapacitated.
Berserk is a more permanent alternative that forces them to use only their physical attack instead of their other abilities like Curse, etc. but their base physical lowers defence stats by 5% on the target each time so be careful if going this route.
Confu can turn them on the boss for you, but they may hit themselves and remove Confu (I think this can be prevented with the use of Resist so that they remain Confused) or accidentally kill the other and prompt Whirlsand.
Izanami has an ability called Blessing which buffs the main boss' stats by 15% so this one should be incapacitated first if you need to choose. Silence should block this.
You can open the battle strong with Alexander to reliably inflict Stop on both the flying cohorts (it also can't kill them as they have 10,000HP and damage cap is still 9999 in this mod) but you'll need to be careful about using multi-target damage after that. Follow that Stop up with Slow and Sleep to keep them down for a long time.
As for the boss itself, Paralysis works so Cross Slash or some Dazers can help if you need a breather. Holy element is the way to go damage-wise, as Poison doesn't work against undead enemies. Elements to defend against are Holy, Fire, and Water (2 attack use Water). Shout is also used; this can be blocked using Elemental + any Bahamut summon in your armour. You likely won't be able to block them all, so pick which attacks are causing the most trouble and block those:
Uzume Fall: Holy (multi-hit, inflicts Darkness), base 8 x10 hits
Leviathan: Water (same as summon, inflicts Silence, ignores MDef)
Kagura: Shout, Confu/Silence, hits all, base 40
Sunyata of Fire: Fire, Inflicts Slow, swaps target Row, base 55
Sunyata of Water: Water, Removes Sadness/Fury, base 72
One thing to watch for is buffs that the boss will use. Sunyata of Wind gives Haste and Auto-Crits, so have Dispel handy to remove that ASAP. Sunyata of Earth is a Wall + Reflect, so Debarrier can take care of that. For status defence, Confu is likely the priority but if you block the elemental damage of an attack then you don't need to worry about the statuses attached to that attack.