Fly to Snowbelle City . Take the south exit out of town, leading to Route 20 - Winding Woods . Go south out of the initial area (with the mossy rock) into to the next area, then west, then north past the twins. Now head towards the west exit, past the fairy tale girl, but take the exit leading south just before it.
Go south in the next area with the big tree. Now continue south and when you see the path leading south, then west, take it. Head through the exit at the end and you'll be in the Pokémon Village . Here, head west and a little south so that you find the ramp leading up west.
Head up it and move over to the nearby water. Use Surf on the water. Follow the water north to the waterfall at the end. Now move onto the land to your east. From there head east until you reach the cave . No one will be standing in front of it blocking your path anymore. Sweet! Head on through!
Legendary Pokemon Battle
Obviously, you'll want to catch this guy - short of trading, you'll be lucky to find this Pokémon again. The sure-shot method would be to use the Master Ball .
The other way? Teach a Pokémon to use Thunder Wave or Stun Spore and False Swipe . Either of the first two moves can be used for Paralysis, which boosts catch rates. False Swipe is a 40-Power Normal move that won't kill. (If you can't use it, try moves that deal less damage than normal: it's riskier, though!)
You can try using False Swipe for two turns after Paralysis is induced to try and see how well the quadruple-rate Quick Ball will do on this fourth turn. If it fails, then just lower the Pokémon's HP to 1 and then begin shooting Ultra Balls at it.
Keep track of turns, though - after 20 turns pass, the Timer Balls will begin to be more effective. They have a x3.0 catch rate at that point (Ultra only is x2.0), but if you're low in number, you may want to wait some more until they're up to x4.0 later. Dusk Balls also work, since you're in a cavern.
(For the record, status-wise, Sleep and Frozen are 33% more effective to the catch rate than Paralysis . However, neither stat is permanent without curing, and no move causes Freezing without damaging the Pokémon.)
Once it's caught, head back outside. Feel free to do whatever you want!