Go to Eterna City and talk to an old man called The Underground Man in the house next to th Pokemon Center
go underground and mine until you find one
Piplup is obtained as a starter in Diamond and Pearl or through trades.
lost towerfloor2 and underground rare in underground
No sorry
You can but not many
trade it from soulsilver
No. The underground is only in Pearl, Diamond and Platinum.
you can only find a cranidos on diamond and a shieldon on pearl
Sorry, but Entei can not be obtained in Pokemon Diamond and Pearl itself, you must transfer it from old GBA games and use the Pal park in DP to catch him.
You have to migrate from Sapphire or Emrald.
Put Honey on a tree
You can't mine underground in either Pokemon Heartgold or Soulsilver, but you can mine underground in Pokemon Diamond, Pearl, Platinum.