Actually, the Haban Berry Kyurem can't beat Mega-Gyarados
252+ SpA Kyurem Draco Meteor vs. 0 HP / 0 SpD Gyarados-Mega: 186-220 (56.1 - 66.4%) -- guaranteed 2HKO
-2 252+ SpA Kyurem Draco Meteor vs. 0 HP / 0 SpD Gyarados-Mega: 93-111 (28 - 33.5%) -- 0.1% chance to 3HKO