Offensive-minded trainers are likely to be disappointed with Shaymin, Seed Flare aside. As a sweeper, Shaymin is very good, but not great. Still, it can lay the smack down on its enemies with Seed Flare, Psychic, and Hyper Beam. Seed Flare should almost always be included in any Shaymin set (otherwise you're almost always better off using Celebi). Psychic counters Heracross and Infernape. Hyper Beam is our preferred choice for Salamence. The last slot should be used for Hidden Power.
Above: Use Hyper Beam of HP Ice to dispatch Salamence
Hidden Power Fire is our favorite for this set, as it rounds out Shaymin's coverage nicely. You can opt for Ice if you don't like Hyper Beam's two-turn nature (and many of you don't). If you choose Ice to dispatch Dragon types then you can implement Rest to take advantage of Shaymin's Natural Cure ability and increase its survivability. Lastly, Ground is another viable choice for Hidden Power, taking out opposing Fire and Steel Pokemon (bye bye Heatran!).
Our favorite way to use Shaymin is as an annoyingSub/Seed Pokemon. Substitute and Leech Seed can be used to stall for time to ensure that Seed Flare slashes your opponent's Special Defense. Hidden Power Fire gives Shaymin another solid offensive option. More importantly, it counters Whirlwind Skarmory. While this set isn't as offensively diverse as the one we mentioned earlier, Leech Seed and Seed Flare can wreak havoc on your foe, forcing them to switch Pokemon.