Instructions
Complete the game by defeating the Elite Four and the Pokemon Champion. This will open up the section of the Safari Zone where Miltank is located.
Travel to the Safari Zone on Route 121 and pay the attendant.
Go to the right side of the Safari Zone, past the green hiker. Walk in the grass until you encounter a Miltank.
Throw as many Safari Balls as you have at Miltank in order to capture it.