You can catch Minun in the Pokemon Mansion Trophy Garden, on Route 212. You must already have the National Dex. Talk to Mr. Backlot, and he will tell you about a certain Pokemon that was not in the Trophy Garden before. This exotic Pokemon changes daily, so keep checking until you get Minun.
Another way to get Minun would be to migrate him from Pokemon Emerald, via Pal Park.
Hope this helps =)