Harry thinks he needs to wear Ethnui down ‘to Harry her’ that means physically and emotionally so his will is the stronger. I think this is where Mac comes in, Harry has skirted over Lugh’s father, describing him as a ‘handsome Tuatha’ so is Harry going to go all Larry Fowler over Ethnui? “Ethnui, as a surprise we have in the studio your long estranged husband, who seduced you in prison after a millennia of confinement”.
Physically? there are likely only three weapons on the field which can injure Ethnui the two swords and the Athame. Gungenir is with Odin, and whilst Etri might have Mjolnir, unless Hendricks is Thor, not an option. I doubt even the rune axes of the Valkyries would have much impact. The swords are also most likely to get close to Ethnui, and allow their wielder as to be able to survive the Eye under the protection of the White God.
I also have this idea that Harry gets the eye out and turns it on the Fomor like the head of Medusa, decimating their forces, eventually taking it back to the armoury with the rest of his superweapons. I wonder if Alfred could wield it?