If you find articuno in x/y:
Articuno w/Ice Plate
Moves:
-Blizzard
-Ice Beam
-???
-???
If you did NOT find articuno in x/y:
Mewtwo w/mewtwoite y(x is ok)
Moves:
-Ice Beam
-Recover
-Psychic
-???
If you do NOT own x NOR y:
Regice w/ ice plate
Moves:
-Ice Beam
-Ancient Power
-???
-???
OR
Salemence w/salemencite
Moves:
-Outrage
-Dragon Pulse
-???
-???
(I am not sure if salemence's moveset is legit off the top of my mind)
Hope I helped!