Nice shaymin! This Pokemon is perfect for competitive battling, so i'll give you two sets- one for regular shaymin, one for shaymin-sky (see bottom for how to change form).
Shaymin@Life Orb
EVs: 4 HP/252 SpA/252 Spd
Timid Nature
-Seed Flare
-Earth Power
-Dazzling Gleam/Psychic/Air Slash
-Rest/Synthesis
This set capitalizes on all of shaymin's special attacks. Seed Flare and Earth Power are standard moves, with the third slot for coverage of choice. Usually, I'd suggest hidden power fire, but you have 6 ivs. Rest is used to regain all health, and shaymin can switch out to heal sleep due to natural cure. Synthesis can be used so that shaymin does not have to switch to get rid of sleep. Obviously, life orb is there for more power.
Shaymin-Sky@Life Orb
Trait: Serene Grace
EVs: 4 HP / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
-Air Slash
-Earth Power
-Seed Flare
-Synthesis
This set completely abuses Shaymin-Sky's ability, as an 80% chance to drop spD two stages from seed flare and a 60% chance to flinch. Earth power is there for coverage, and synthesis is there for healing. Alternatively, Earth Power, Synthesis, and Life Orb can be dropped in exchange for Substitute, Leech Seed and Leftovers, which would make this a subseed set.
"In the Berry Master's house in Route 123, you will find this person talking about the Gracidea flower from Sinnoh. If you show him a Shaymin, he will give you the Gracidea which allows you to change Shaymin's form to its Sky Forme from 5am to 8pm every day."
http://www.serebii.net/omegarubyalphasapphire/formchange.shtml
