I think calling for a Xerneas ban is a bit hasty at this point. Dusk-Mane was by far the best counter, yes, but there are still answers to it.
First, as you mentioned there's Unaware Solgaleo. This is essentially Dusk Mane 2.0, and you already mentioned it so I'll keep this brief.
252+ SpA Tinted Lens Xerneas Moonblast vs. 248 HP / 252 SpD Solgaleo: 146-174 (30.6 - 36.4%) -- 2.9% chance to 3HKO after Stealth Rock and Leftovers recovery
Xerneas does not win this. Even with Twinkle Tackle, which would be meh because then you're not Z-Geomancying.
252+ SpA Tinted Lens Xerneas Twinkle Tackle (175 BP) vs. 248 HP / 252 SpD Solgaleo: 270-318 (56.6 - 66.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Second, Volt Absorb Ho-Oh. Much less reliable, but still workable.
Ho-Oh @ Leftovers
Ability: Volt Absorb
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Recover / Roost
- Whirlwind
- Sacred Fire / Toxic
- Brave Bird / Toxic
+2 252+ SpA Tinted Lens Xerneas Moonblast vs. 252 HP / 252+ SpD Ho-Oh: 180-214 (43.2 - 51.4%) -- guaranteed 3HKO after Leftovers recovery
Theoretically this can stop anything but Ingrain Tinted Lens or HP Rock Xerneas. It needs Stealth Rocks off the field, and you need sufficient offensive pressure that you can kill Xerneas some other way, because phazing alone will eventually lead to a last mon sweep.
Third, Marshadow. Clearly not a defensive answer, but check this out
Marshadow @ Focus Sash
Ability: Magic Guard
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Spectral Thief
- Close Combat
- Endeavor
- Shadow Sneak
This things has insane revenge killing potential if you make sure not to let it get chipped, which thanks to Magic Guard isn't hard. It can either steal Xerneas' boosts forcing it to either use its base stats or boost again over two turns or Endeavor + Shadow Sneak it.
Fourth, Unaware blob.
252+ SpA Sheer Force Xerneas Focus Blast vs. 0 HP / 252+ SpD Blissey: 220-260 (33.7 - 39.9%) -- guaranteed 3HKO
You guys know what this thing does.
I'm not necessarily saying that Xerneas is balanced, but answers do exist to it. A lot of them are a bit matchup based and win or lose against different abilities. For instance Mold Breaker and Clones beat both Ho-Oh and Blissey (well Confide Blissey can put up a fight) while Magic Bounce + Rest walls several of these sets in return (Ho-Oh and Blissey again :X). In my personal opinion it's too soon since the last ban to call for another ban, although I would love to hear other people's input. I definitely won't be making hasty decisions though.