Is it a helicopter?

Use only Metal-type energies!!

• Shaymin goes in the back, without energies (it needs Grass ones), just to heal with its ability.
• Dialga goes in the front, tanking hits. Use your Potions, Pokémon Center Ladies and maybe a Giant Cape to keep it alive for many turns, hopefully until you can get four or more energies on a Yanmega and/or on Dialga itself for Heavy Impacts.
• The Yanmas (or Yanmegas) behind recieve Metal energies from Dialga's Metallic Turbo. Once they are ready, use the two energies your Dialga has to retreat it and send Yanmega in. It deals 120 damage every turn (140 after using Red), and with a Giant Cape its 160 HP make it barely survive a Darkrai's Nightmare Aura + Giratina's Chaotic Impact combo.
• Just remember to use Shaymin's ability once per turn, and to give Yanmega any energies you have once it is active - its attack burns one energy every time it's used!

Using this deck I reached Mega Ball 2 in just two days after downloading the game