So, the nature "calm' is a bad for Shaymin stats, eh?
Decreases: Attack power(Shaymin is more of a Special Attack pokemon ex:"Seed Flare" and more likely to baton pass. You think?)
Increases: Special Defense (So shaymin won't get knocked out by OHKO Special Attacks?)
Add to that my Shaymin "Loves to eat" So HP and some trainging @ the Old Chateu should round off its SP Atk pwr.