West of Village Bridge you find Route 11.
Explore the Route
On the bridge to the west, you find PKMN Breeder Magnolia, who has a level 41 Emolga and a level 41 Altaria. She will battle you whenever you come back to this area.
Inside of the trailer, there is a PKMN Ranger who will give you some Favored Mail.
Just past the trailer, there is PKMN Ranger Crofton, who has a level 42 Tympole and a level 42 Palpitoad. He gives you an Aspear Berry after you defeat him.
Walk around the area to the left of the trailer truck to get an Xtransceiver call from Curtis or Yancy.
If you Surf in the water south of the bridge here, go to the grass to the left and walk west to find a Full Heal.
There is more to find in Route 11, but you can't reach it until you have Waterfall.
After you go west across the bridge, you will see Virizion up on the cliff. It will jump down in front of you. If you're ready, you can talk to it to battle it!
To the left of Virizion, use your Dowsing Machine to find a Max Potion in the grass. Opelucid City is to the west.
Here in Route 11, you can battle Virizion, who is currently at level 45. Feel free to go west into Opelucid City to prepare first.
If you knock Virizion out, you will still get a chance to catch it, but you have to wait until after you have beaten the game.
I don't recommend using your Master Ball on Virizion.
Virizion has a very low catch rate, which means that when you throw pokéballs at it, they have a low chance of succeeding, so you will most likely have to throw a lot of pokéballs at it until you catch it.
You can increase your chance of catching Virizion by lowering its HP as much as possible. If you have a pokémon with False Swipe, you can use that to lower Virizion's HP to 1.
Also, Drifblim/Drifloon, and Litwick/Lampent/Chandelure, have very good resistances to Virizion's attacks, so you might want to use one of them to lower Virizion's HP.
You can also inflict Poison, Burn, or Paralysis on Virizion to improve the chance of pokéball success. Better yet, inflict Sleep or Freeze, because these two status ailments improve the chance of success even more than Poison, Burn, or Paralysis.
The type of pokéball most likely to succeed is a Dusk Ball, but only if the battle began between 8PM and 3:59AM. Otherwise, use Ultra Balls.
The maximum chance of pokéball success on Virizion (that is, if you inflict Sleep or Freeze, lower Virizion's HP to 1, start the battle between 8PM and 3:59AM and use Dusk Balls) is only about 8%.
Continue to Opelucid
When you're ready, go west to reach Opelucid City. If you don't fight Virizion, it will wait for you in Route 11.