Since Sky form Shaymin is fast and ice moves are 4x effective to him you'll need faster Pokemon than him with any good ice move or a Pokemon with good attack state and got ice shard.
An example for a faster than Sky Shaymin with good ice moves is:
Ash Greninja(Ice beam learned by TM)
Mega Lopunny(Ice Punch egg move)
Mega Alakazam(Ice Punch egg move)
Mega Gengar(Ice Punch egg move)
There is more but I can't write them all.
Speed attack or normal form Deoxys(Ice beam learned by TM)
An example of a Pokemon with good state attack and have Ice Shard is:
Last way is to bring any bulky in sp.defense Pokemon.
Heatran can 1 hit K.O the Shaymin with a fire move.
Lugia is VERY bulky in sp.defense so Shaymin will be easy for it.It can use Ice beam and finish it up.
Regice is bulky too and uses icy wind to end up the Shaymin.
But since Regice Lugia and Heatran are hard to get you can use:
Bronzong is got good at sp.defence and its steel type will help it.It can use 2 clam mind and then recover by rest and then use toxic and keep recovering until the Shaymin faints.
Sylveon will simply use Draining Kiss until the Shaymin is dead.
Thanks for reading and hope this will help you out.