well the avatar stills one hell of fighting power let's recapt your list
marneus calgar, basically a clash of demigods
Epistolary of the blood ravens(basically chapter master aplicant
Garbiel motherfukinng angelos
aganist what? a powerhouse of marine squads?a great unclean? he maybe a godaam goodly warengine but everything has its limits
and laslty maugan ra it's if not the first the second most "heretical" of phoenix lords
If the idea made blood (and lava and molten steel) of the Wrath Incarnate of an entire race whose Lust Incarnate spawned a new god is loosing against a chapter master applicant - yeah that's pretty sucky for a performance. There are a bajillion space marine chapters out there and jsut as many aspirants ... but this is basically Wrath Incarnate.
To continue the list -
he was trampled to death by a dozen Fexes
strangled to death by Fulgrim (how do you strangle a being that doesn't need to breathe?)
possessed by a Kepper of Secrets
I can see this going very well then going very badly the moment they start gaining an advantage.
Better plan, find the Blood Ravens steal their hairgel, coat yourself with said hairgel and bam. Epic hair and armor that rivals even the God-Emperor himself.
That or find the nearest Imperial Ground Fodder regiment and with any luck the nids would go for them instead of the Eldar because rule #192 of the grim dark world of grim dark, Guardsmen must die horribly first no matter how illogical or inconvenient it may be.
The good news is I don't True Darius has the same lame-on for the Avatar that GW prefers.
It's possible that the psychic hate-quake the Avatar could generate might shake awake any forgotten Wraith units and maybe mobilize the actual physical worldcraft _itself_.
The spirits of the dead power the Affinity Circuit after all, and it's conceivable that in desperation they could take a more active role in preventing it's utter consumption.
Or maybe those would be it's death-throes.
Either way, not like things have much worse to get.