The easyest way is to wait for a mystery gift event.
Actually that applies to Soulsilver version.
Heartgold can catch Kyogre in the Embedded Tower and nowhere else.
But you will need to trade or wait for a mystery gift for Groudon.
Only a Kyogre (caught in the Embedded Tower-Heartgold)
You will need the Blue Orb first talk to Mr. Pokemon.
+
a Groudon (caught in the Embedded Tower- Soulilver)
will let Prof. Oak give us the Jade Orb so that we can catch
Rayquaza I think in the Embedded Tower
you cant catch kygore in pokemon platinum unless you migrate from pokemon sapphire or emereld
you catch the hoenn legnds kygore groudon and rayqaza
You're better off learning how to fly irl then catching a kygore on ruby.
You can get groudon in Pokemon soulsilver and kygore in Pokemon heartgold (both are found in the embedded tower on route 47).
Trading from heartgold to soulsilver
Groundon or Kygore and Rayquaza, Suicune, Raikou, Entei, Ho-oh and lugia
You can't catch Rotom in heartgold.
Dialga Palkia, Rayquaza,Kygore and Groudon
you cant catch him in heartgold you can only get him in soulsilver
Trade Kygore from Heartgold then talk to proffeseur Oak with groudon and Kygore in your party
In HeartGold you can catch Kyogre.
You can only catch Vulpix in SoulSilver, not in HeartGold. You can catch it on route 36 and route 37.