So, what Dan can against Evernight, who, unlike March, will not hesitate to use methods that could kill or permanently incapacitate her enemies, such as Possession, Existence Erasure, Transmutation and others?
Depends, Dan's own possession (and resistance) is accepted as 5 layers (It's effects include biological manipulation on a molecular level, effecting the subconscious, emotions and such), his energy naturally has this effect so if she's hit and lacks the resistances, it could be a loss. Transmutation can be resisted since ectoplasmic energy alters matter on a molecular level, however he doesn't resist Existence Erasure so that's a solid win method.
I'm busy with stuff so I'll simply explain Dan's fighting method...
Passive: This Dan is on a timer, in about 24 hours he'll destroy the entire multiverse including himself in an implosion, so if she can survive in the void of nothing she'll win simply by lasting that long. He's passively messing with space and time, which includes potentially yeeting people to different periods of time (BFR), disabling tech (don't know if she uses any), and altering the landscape. He has no control over these effects.
Tactics: Dan himself opened with Time Stop (snapping his fingers), if she resists it then he'll simply do the same thing he did against Danny, use his Ghostly Wail (a powerful sonic scream that's city wide, and negates low-godly level regeneration), shooting multiple ectoplasmic energy blasts (their natural effects are mentioned in the Ghost Physiology page), and close combat (punches and grabs), Telekinesis to stop projectiles.
Basically it's pretty simple, if she resists BFR and can survive Low 1-C annihilation (that can kill other Low 1-C characters) then she's in no danger from his passive existence. As for his abilities, she's safe from the layered passive effects of ectoplasmic energy if she dodges his blasts unless he just covers her in his energy then it's GG unless she's got layered resistance to possession, biological manipulation, mind manipulation, disease inducment. She's safe from Time Stop if she resists, and that's pretty much it.
I don't have time to look at her profile but I'll vote for Evernight if she opens with Existence Erasure since Dan can't stop that (unless it requires contact), but from what I see she's got the senses and the interaction to effect him so if she's skilled enough to dodge and fight she theoretically can just beat him up and undo the unstable fusion. Plus if Information Type 2 and Conceptual Manipulation Type 1 are potent enough he's cooked. I'll leave it up to y'all, I got stuff to do, just dropped by to explain what this Dan does in combat real quick.