Speak softly, sit down so you are close to its level and therefor not as intimidating, and have patience. [[Rose has dealt with one or two extremely shy Pokemon in her time.]]

If you've any food or berries, you can try setting those out for it, especially if it has yet to eat. And most importantly, let it come to you. Don't try to touch it until it has made itself acquainted with you and your smell and seems comfortable. That would be my suggestion.