Go to Kanto and take Drudigon to Bill's secret garden. After feeding Bill the Fresh Water you were given in the final gym, he gives you the keys to the car in front of the S.S. Anne. Move it to find Mew underneath holding the H4x0Rs Stone. Use the stone on Druddigon while fighting MissingNo, and he'll evolve.
"My head's filled with only LIQUID OOZE now!" -My trainer card
It evolves at precisely lvl 67, if you pass this by, you're out of luck. At that lvl take both Reshiram and Zekrom to battle Landorus. Druddigon has to be the pokemon in battle when you capture KO Landorus. Then take it to the top of Celestial Tower at midnight, and ring the bell. A wild Lampent will appear. You have to steal it's item, the Red Talon. Take this item to Iris, and she'll explain it's use. Travel to Kyurem's cave, and the draco stone will be there. Now, lvl it up near the draco stone, and it'll evolve.
Note: Once you get access to the draco stone, you can evolve any dragon type pokemon here by leveling them up nearby(Except Druddigon, which has to be lvl 67).
No no no you all have it wrong. To evolve Druddigon you must do the following: 1. Battle Champion Alder using Druddigon, and only Druddigon, you only get one chance, and if it faints you can't revive it. You must win only using Druddigon. This maximum level your Drudiggon can be is precisely 52, no more no less. 2.After you win Alder will say "Man dude, your Druddigon is intense, I thought they all sucked" then he will proceed to the champions room. 3. In the champions room rests a level 20 Keldeo that you must defeat in one turn, you may not capture it or Druddigon will not evolve. 4. After you defeat it the standard ending credits will play. When you continue your game rather the being in your home Alder will take you back to his house, which you will proceed to wake up in bed next to him. He will then say "Man that was one intense match last night..." Your character will then run outside and see Jirachi floating in front of the house. Jirachi will grant you one wish and it is either to evolve Druddigon, or he will give you a free GoLURK MOAR
And there you have it, hope this helps.
GT: The Lunescent. Black FC: 0389-8395-5178 Name: Hidori