Er, if Shuckle goes first with swagger, Gyarados hits itself and shuckle uses toxic, Shuckle will win.
If Gyarados goes first with ice beam, Shuckle survives but has minor than half the HP left, It can use rest or swagger or toxic. If it uses rest, Gyarados can knock it out when its asleep, Because shuckles snore cant knock it out enough fast. If it uses toxic, Gyarados becames poisoned but knocks it out. If he uses swagger he can continue with rest but when gyarados snaps out it will still have increased attack. The one that starts wins. But Gyarados often has high speed, so i vote for him.