Go to the spear pillar and make sure you have caught BOTH dialga and palkia and make sure you have them in your party then you go to the top then use an escape rope then you go back up but before you finally out the the cave to the top of mount cornet/spear pillar stop at the entrance to the spear pillar and make sure you have done EVERYTHING CORRECTLY then you go out the cave to the spear pillar and there should be the portal as there was with palkia and dialga but this time it should be red then you go to it click A it should say a creature is living in this portal and then it will say it is groudon and then it will screech and you'll have to battle it ,it is a level 50 so be prepared but you can also use A action replay or game shark
