Route 655

Pokémon:
Lv. 14-16 
Patrat - 25% 
Pidove - 25% 
Oddish - 15% 
Bellsprout - 15% 
Sawk - 10% 
Throh - 10% 

Items: 
Burn Heal x1 
Net Ball x1 
Smoke Ball x1 
Super Potion x1 
TM45 
Pokémon Egg (Talk to man inside Daycare) 
Everstone (Show Pokémon after it hatches) 
Lucky Egg (Show Pokémon after it fully evolves) 

Trainers: 
Battle Girl Lee - Sawk Lv. 16, Throh Lv. 16 
Pokémon Breeder Galen* - Sewaddle Lv. 16, Patrat Lv. 16, Bibarel Lv. 16 
Pokémon Breeder Brooke* - Misdreavus Lv. 16, Luxio Lv. 16, Oddish Lv. 16 
Lass Serena - Kricketune Lv. 16, Furret Lv. 16, Ekans Lv. 16 
Lass Isabel - Nincada Lv. 16, Pidove Lv. 16, Geodude Lv. 16 
Youngster Wes - Zigzagoon Lv. 15, Riolu Lv. 16, Beedrill Lv. 16, Beldum Lv. 16 
Youngster Cody - Whismer Lv. 16, Bellsprout Lv. 17 
*Get number 

Holy trainer battles, batman! Looks like you've got some work to do. Free up a slot and get that Pokémon Egg for a Timid Togepi. I say throw an Exp. Share on it when grinding, since it might prove to be a great addition to your team. Once you've gotten through the marathon of trainers, you'll run into mystery man again. Oh boy. 

Mystery Trainer ??? 
Stunky - Lv. 20 
Absol - Lv 22 

Not tricks here, so he should be easier than the first time you fought him. You'll get a Macho Brace for the trouble. 

Keres 

Items: 
Pecha Berries x2 (Talk to nurse in Pokémon Center) 
Antidote x1 
Super Potion x1 
Miracle Seed (Hidden behind tree all the way southwest) 
TM44 (Talk to man inside second eastmost house) 
TM49 (Talk to girl in house below gym) 
TM84 (Requires Strength) 
Good Rod (Talk to fisherman in the eastmost house - Requires Old Rod) 
Quick Claw (Talk to hiker in route gate) 

Sure is wet around here. Too bad you can't go past the guards until you get your third badge. If I haven't made it obvious enough, you're going to need stuff that cures poison for the next gym. 

Third Gym 

Trainers:
School Kid Calla - Nidorina Lv. 16, Zubat Lv. 16 
School Kid Scotch - Nidorino Lv. 16, Ekans Lv. 16, Oddish Lv. 16 
Fisherman Larkspur - Qwilfish Lv. 17 

You have to find switches in stumps to create a bridge. Third one will take you to the gym leader. (Parenthesis is hard mode.)

Gym Leader - Spurge
Qwilfish - Swift Swim (Poison Point) - Sitrus Berry - Lvl 24 
•Water Gun 
•Toxic Spikes 
•Rain Dance (Minimize)
•Venoshock 
Crobat - Inner Focus - Sitrus Berry - Lvl 24 
•Bite 
•Quick Attack (Wing Attack)
•Confuse Ray 
•Venoshock 
Nidoking - Poison Point - Sitrus Berry - Lvl 25 
•Focus Energy 
•Horn Attack 
•Double Kick 
•Venoshock 
Obtain TM09 - Venoshock 

Remember: Venoshock doubles if you're poisoned. Putting those Pecha Berries on your best mon will save you a lot of trouble.

Now that the guards are gone, you can go through the gate. But what's this?! A surprise rival battle! 

Alex 
Eevee - Lv. 20 
Kirila - Lv. 22 
Pidgeotto - Lv. 22 
Starter - Lv. 24 

Damn, not even as strong as the gym leader. On the bright side, you have Strength now! After this battle you're not gonna see Alex for a while. Oh well. Next! 

Panpan Forest 

Pokémon: 
Lv. 15-17 
Pansage - 20% 
Pansear - 20% 
Panpour - 20% 
Venipede - 15% 
Murkrow - 15% 
Drifloon - 10% 

Items: 
Ether x1 
Cheri Berries x2 
Ultra Ball x1 
X Accuracy x1 
Burn Heal x1 
Balm Mushroom x1 (Hidden in mushrooms) 
Hyper Potion x1 (Requires Cut) 
TM22 (Requires Cut - Hiding behind trees) 
TM52 (Requires Surf) 
Silk Scarf (Requires Surf) 

Trainers: 
Pokémon Ranger Lewis - Patrat Lv. 18, Houndour Lv. 18 
Pokémon Ranger Veronica - Houndour Lv. 18, Zubat Lv. 18 
Male Grunt x2 - Poochyena Lv. 17, Murkrow Lv. 18 
Female Grunt x2 - Purrloin Lv. 18, Murkrow Lv. 18 
Female Grunt - Purrloin Lv. 17, Shinx Lv. 17, Skitty Lv. 18 

This forest is bountiful with items. Go collect what you can while battling grunts along the way, and you'll run into an interesting scene that leads to a battle. 

Leona 
Skitty - Lv. 20 
Liepard - Lv. 22 
Luxio - Lv. 22 
Persian - Lv. 28 

That's quite a level jump! It doesn't have anything special other than that, so just beat it senseless. You'll be rewarded with HM03. 

Route 656 

Pokémon: 
Lv. 16-18 
Growlithe - 25% 
Skitty - 25% 
Minccino - 15% 
Shroomish - 15% 
Drowzee - 10% 
Electrike - 10% 

Items: 
Ultra Ball x1 
Parlyz Heal x1 
Heal Ball x1 
TM41 
Ether x1 (Hidden in flowers) 
Blackglasses (Talk to guy in route gate) 

Trainers: 
Doctor Jules - Audino Lv. 18 
Nursery Aide Hollie - Rattata Lv. 17, Electrike Lv. 17, Pansage Lv. 17, Pikachu Lv. 18 
Preschooler Greg - Pansear Lv. 18 
Preschooler Eva - Panpour Lv. 18 
Twins Ally & Amy - Cottonee Lv. 18, Petilil Lv. 18 
Pokémon Ranger Eliza* - Marill Lv. 17, Growlithe Lv. 18 
Pokémon Ranger Keith - Minccino Lv. 18, Drowzee Lv. 18 
*Get number 

Heal with the good doctor after you beat him. You can do a double battle with the twins if you want. Going west takes you to town, but we're going to take a detour south first. 

Berry Farm 

Items: 
A ton of berries 

Beat up the grunt and you'll be free to pillage the farm. Talk to the lady inside and she'll give you a different berry depending on the day of the week. Her husband will also give you berries as thanks. Berries! 

Meliae 

Items: 
Super Repel x1 
X Defend x1 
Silver Powder 
Rare Candy (Talk to girl in Poké Center) 
Sun Stone (Talk to old man walking outside) 

Pokémon: 
Karrablast/Shelmet (Trade with twins) 
Lapras (Talk to the old man in the southern house) 

To the northwest is the Natural Park, where you can enter the Bug Contest on Tuesday, Thursday, and Saturday. First place is a random evolution stone. 

Natural Park Pokémon: 
Lv. 10-20 
Karrablast - 25% 
Shelmet - 25% 
Venonat - 15% 
Heracross - 15% 
Scyther - 10% 
Pinsir - 10% 

Say, is that your rival in front of the gym? You know what that means! 

Emil 
Pansage/Pansear/Panpour (Same type as your Starter) - Lv. 22 
Eevee - Lv. 24 
Nidorina/Nidorino - Lv. 24 
Starter - Lv. 26 

Wow, the team has doubled! This is also the last time you'll be fighting this rival for a while. Remember to stock up on potions and revives! 

Fourth Gym 

Trainers: 
Lass Lisa - Butterfree Lv. 18, Dustox Lv. 20 
Youngster Bud - Spinarak Lv. 18, Beautifly Lv. 18, Kricktune Lv. 18 
Pokémon Breeder Milinda - Swadloon Lv. 22 

Gym Leader - Debbie 
Accelgor - Unburden - Sitrus Berry - Lvl 26 
•Double Team 
•Struggle Bug 
•Mega Drain 
•U-turn 
Heracross - Guts - Sitrus Berry - Lvl 26 
•Night Slash 
•Bug Bite 
•Brick Break 
•U-turn 
Scizor - Technician - Sitrus Berry - Lvl 28 
•Bullet Punch 
•Quick Attack 
•Pursuit 
•U-turn 
Obtain TM89 - U-turn 

Fast hit-and-runs are to be expected. Hope you can keep up!