At level 45, Virizion is in a position to be an excellent addition to your team, perhaps even moreso than Cobalion.  As a Fighting/Grass pokemon, its type combination is only found elsewhere in Breloom, with whom it shares little else.  While Helping Hand does absolutely nothing for it, its Sacred Sword and Retaliate attacks will dish out considerable damage, and Giga Drain has a high enough power that it barely matters that the move uses Special Attack- it will keep Virizion out of catchable range a fair bit of the time.  Given your team is unlikely to be beyond level 45 at the moment, your best bet is most likely to walk around the thing and come back in a little bit, when you're more prepared.  Whenever you do confront it, though, you'll pretty much have to have a way to put it to sleep or freeze it so that it can't heal itself with Giga Drain.  Freezing it gets dicey, since it's a Grass type, but a bit of odd tactics, such as using a low-power Special attack that can inflict Freeze with a Sneasel or Weavile (whose special attack is on the poor side) may work.

When you return later it will be level 65 and have a much more powerful (but much more narrow) moveset.  While level 65 Cobalion retains Sacred Sword, it has dropped all three of the earlier moves in favor of Swords Dance, Work Up, and Quick Guard.  With only a Fighting-type attack, it becomes very vulnerable to ghosts and ceases to be as much a danger for Ground or Water pokemon- but with Swords Dance to skyrocket its attack, disabling or taking it down immediately becomes a much more imperative goal.  Quick Guard can help it avoid priority attacks, not that that should be a concern for it, but your main problem here is Swords Dance and the hellacious offense it grants the deer.

With Virizion revealed, you can hop into the water from the bridge and duck around the fence to the South to pick up a Full Heal off the riverbank.  Finally, enter the grass North of the road and look in the Northwest corner to pick up a rare Max Potion hidden in the dirt.

That's everything you can do on the route for now, so head off to the West through the gate building.