To get to Route 8 from Icirrus City, go east from the Icirrus Pokémon Center, then go down the stairs that are next to the Parasol Lady. Once you're at ground level, go to the right of the stairs that you came down, and then go up. You'll pass under the bridge and be on Route 8.
Pokémon that you Can Catch Here
In every season but winter, you will randomly encounter wild pokémon if you walk on the puddles. These pokémon will be Palpitoads, Shelmets, and less frequently, Stunfisks. In winter, however, the puddles are frozen, and you'll slide across them without any pokémon encounters.
If you Surf here, you'll encounter Stunfisks. Bubbling spots on the water will be either Stunfisks, or, rarely, Seismitoads.
Recommended Pokémon to Bring
I recommend bringing Ferroseed, Leavanny, Deerling, Golett, and Emolga.
Route 8 in Winter
As mentioned above, the route 8 puddles are frozen during the winter. This makes it more difficult to get around. Once you enter the route, you will have to slide west from the entrance, then up. From there you can reach Pokémon Ranger Lewis.
To get into Moor of Icirrus during the Winter, you will need to bring a pokémon that knows Surf, and use it to go across the water behind Pokémon Ranger Lewis. Get out on the eastern side of the water, then slide east and you will be able to get into the Moor. If you want the hidden item near Annie during the winter, be sure to get out on the eastern side of the water, but at the bottom corner of the water. That way, when you slide east, you will bump into a tree rather than ending up on grass, which will allow you to slide up into the area that has the hidden item.
As far as I know, there is no way to get the hidden item near the Icirrus City exit of Route 8 during the Winter.
Going through Route 8
The first trainer that you will encounter is Pokémon Ranger Lewis, who has an L34 Cubchoo, Palpitoad, and Maractus. Against Cubchoo, I recommend using Frillish for the best defense, or Ferroseed for the best attacks (Steel moves). Against Palpitoad, I also recommend Frillish or Ferroseed. For Maractus, I recommend using Leavanny.
Next, you'll meet Parasol Lady Melita. She has an L34 Frillish and Ducklett. I recommend using Deerling against Frillish (attack with Grass or Dark moves), and I recommend using Emolga against Ducklett.
Near the entrance to Moor of Icirrus is Pokémon Ranger Annie, who has an L36 Cinccino. I recommend using Golett against Cinccino.
Near Annie is the entrance to Moor of Icirrus. This is an optional area that you can explore. There are trainers and items in there, but nothing too rare or anything.
Moor of Icirrus
Back on Route 8
If you go south from Parasol Lady Melita, you'll encounter Fisherman Bruce, who has two L34 Basculins. I recommend using Ferroseed against them.
Nearby, you will find Parasol Lady Lumi, who has an L35 Alomomola. Against it, I recommend using Frillish or Leavanny.
Battle with Bianca on Route 8
At the end of Route 8, you will encounter Bianca, who wants to have a battle with you.
She has an L38 Stoutland that knows Work Up, Crunch, Roar, and Retaliate. Use Cobalion if you have it. Otherwise, use Excadrill, Ferroseed, Escavalier, or Klang.
She also has an L38 Musharna that knows Defense Curl, Lucky Chant, Psybeam, and can inflict Sleep. Use Krokorok, Scraggy, Liepard, Zorua, or Zoroark.
Her L38 elemental monkey is the type that is strong against your starter (either Simisear, Simisage, or Simipour.) Its move set is Lick, Leer, and Fury Swipes. If it is Simisear, its fourth move is Flame Burst. If it is Simipour, its fourth move is Scald. If it is Simisage, its fourth move is Seed Bomb. Against Simisear, try using Tirtouga if you have it, or Boldore or Archen. Against Simipour, try using Deerling or Ferroseed. Against Simisage, try using Unfezant or Deerling.
She also has her L40 starter (either Samurott, Serperior, or Emboar.) Samurott's moveset is Revenge, Aqua Jet, Slash, and Encore. Try using Frillish, Foongus, or Leavanny against it. Serperior's moveset is Mega Drain, Slam, Leaf Blade, and Coil. Try using Litwick against it. Emboar's moveset is Rollout, Take Down, Heat Crash, and Assurance. Try using Tirtouga if you have it, or Krokorok, Boldore, or Palpitoad.
After you defeat Bianca, she gives you a Full Restore. You can go check out Tubeline Bridge now.