Is Jim implying that the Erlking used the Dark Hallow Ritual?
I don't believe so, although his wording is a little obscure. My interpretation is that the reason the Erlking is summoned is that the necromancers needed all those spirits, especially the really big old ones. It's what the Erlking does every time he rides. He summons the spirits of the old hunters.
Ethniu didn't really kick their asses, she just thought she did. They needed her to think that so they could get close enough to fight her. Odin let her take Gungnir from him, then when she tried to use it, he turned it against her with a blast to the face which enabled Lara to give her a kick to the back of the head to dislodge the Eye, then weakened and without her main weapon, Harry with Marcone/Namshiel's help come in on clean up and ship her to Demonreach.
That's an interesting interpretation. Except, she was quite a bit stronger than any single one of them and wearing armour that made her nigh-invulnerable...
I can tell you for a fact though that the Gungnir trick that Odin pulled is ripped from 1st edition D&D. Odin is in the Deities and Demigods supplement, and Gungnir has a special trick that means anyone who holds it without Odin's permission is transmogrified into an ant or loses half their hit points.
The Erlking got beaten so badly half his body disintegrated and had to reform, Odin lay unmoving and burned for most of the rest of the battle, and Titania (arguably the next strongest person on the battlefield after Ethniu) was choked out and stuffed head first into the ground. Now, that could be a bluff but if it was it was a hell of one. Just for reference too, when we last saw Odin (as Kringle) pull a bluff, it did seem a bit too easy - this was when Harry out wrestled him and took leadership of the Wild Hunt in Cold Days.
I think everyone got quite lucky that Harry was there, otherwise Ethniu probably would still be roaming around.