I think Sp.Def Dialga with Roar could do the trick sometimes.
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 252 HP / 252+ SpD Dialga: 328-387 (81.1 - 95.7%) -- 12.5% chance to OHKO after Stealth Rock
If you take even a tiny of chip from say spikes or hail prior to facing Xern, you likely get 0hkoed.
You could also use strong priority like Extreme Speed from Choice Band Rayquaza and Zygarde to try and KO it, but this only work if Xerneas is VERY low on health.
252 Atk Choice Band Rayquaza Extreme Speed vs. 0 HP / 168 Def Xerneas: 128-151 (32.5 - 38.4%) -- 98.2% chance to 3HKO
I don't think Mono Dragon has any proper reliable counterplay to Xerneas period.