When you enter, a Lass will ask you to show her a "cute" Pokémon. At this point, you might either struggle to think of one or you'll have something obvious to show, like your partner Pokémon. As far as we know, any Pokémon will do. We didn't have time to try all 153, but if a Raticate or Muk can pass, they probably don't discriminate!
Know Your Match-Ups: Grass
Grass-types are weak to a bunch of things: Fire, Flying, Ice, Bug and Poison. Meanwhile they're strong against Water, Ground and Rock, and resist Water, Electric, Grass and Ground. But note that a lot of Grass-types are also Poison-types, which slightly alter the match-ups.
To start with, go through the flower arches straight ahead. Then turn right at the corner. As you go up the mini slope, the Lass from earlier will be standing towards the right. If you hug the hedges below, you can avoid her gaze.
Oddish is our first Grass and Poison-type. Fire, Flying and Ice are still super-effective; and add Psychic to that list. This Oddish can use Toxic, so Steel-types are also worth considering.
Next, head south along the narrow path, then east. There will be two tree branches that you can remove using Chop Down. Cut down the one along the top, then follow the path north. The path will branch out. If you want to gain more experience, follow the path east to battle a Beauty.
Exeggcute is also a Psychic-type and is thus doubly weak to Bug-types. Bug and Poison-types should be careful of its Psybeam though. Otherwise, stick with Fire, Ice, Flying or Dark-types. If using a neutral Pokémon, watch out for its status condition moves.
From the path branch, follow the path west and around. As you begin to head east, be mindful of a Picnicker just ahead. Again, if you hug the hedge below, you can go past her without a fight.
Things are stepping up slightly, as they start to bring out an evolved Oddish. The same weaknesses apply: Fire, Flying, Ice and Psychic. Definitely try and hit it with super-effective moves before it gets a chance to use its annoying status condition moves.
Continue south from Tina. After the next corner, there's a Beauty waiting to the right. This time, if you hug the hedges above, you can navigate beyond her vision. Hmm, do these ladies need an eye test, or are they just extremely focused?
For Oddish, go with the usual super-effective types or Steel-types to resist its Poison. Weepinbell is an evolved Bellsprout, but shares the same weaknesses as its predecessor and the Oddish line. Similarly, Steel-types can be useful to avoid its Poison Jab.
Head north from Tamia, towards another pair of tree branches. You want to slice down the one on the left so you can proceed west. As you run, you'll end up going past Erika from above. Near the north-west corner, there's another Beauty. Stay near the hedges below if you want to evade her.
Not a lot of variety here, huh? It seems this Gloom only has attacking moves, so that should be an advantage for you.
Now follow the path below. When you reach the bottom-right corner, slow down, as a trainer is watching to the left. It should come as no surprise that you can avoid her so long as you don't stand in the centre of the path.
Ace Trainer Mary
Weepinbell, you should know how to handle. This one doesn't have Poison Jab, but can cause Paralyse. Tangela is new and a rare pure Grass-type. That should make the match-ups really easy for you.
Go up and around towards the left. To the north-west, there's a lonely Lass. Feel free to entertain her.
Yay, three Bellsprout in a row. All seem to have the same moves, which include Poison Jab and ones that inflict status conditions. They're all unevolved, so go crazy with your super-effective mons.
Meanwhile, below, there are tree branches to the left and right. You want to focus on the one on the right. Further south, you'll reach the final pair of tree branches. Let's flip a coin... Nah, the correct answer is the left one. Once it's gone, follow the path upwards and to the east. Ta-da, you've reached Erika!
Watch Your Step!
Please be careful as going south will send you jumping over the ledge back to the start. Only go that way after you've defeated Erika or if you need to run back to heal and/or buy healing items.
Gym Leader Erika
As you'd expect, Erika only uses Grass-types. The main difference is that hers are higher Level. Tangela is a pure Grass-type, so Fire, Flying, Poison and Bug are recommended. Ice-types are good as well. The biggest concern here is Sleep Powder.
Weepinbell is Poison, so the list of weaknesses changes slightly: Fire, Flying, Ice and Psychic. Steel-types are also worth a shot, since they are immune to its Poison Jab and resist its Mega Drain, but we know they're rare.
Finally, Vileplume is a fully evolved Oddish. This gigantic shroom can use Mega Drain to steal health and Moonblast, well, because it can. Fire-types are strongly recommended since they resist both of its attacks. Steel-types too, but they aren't super-effective, unless they're Alolan Sandshrew.
Win against Erika and she'll give your her Rainbow Badge plus TM53 Mega Drain. Compared to earlier games, Mega Drain has received a significant buff, so definitely consider learning it if you have space. A move that can simultaneously cause damage and heal its user can be very useful, as Let's Go Eevee users can probably attest!
Poké Mart Upgrade
After acquiring four Gym Badges, you can start to purchase Ultra Balls. As you may have noticed, it's becoming increasingly difficult to catch wild Pokémon--and these improved balls--the best ones you can buy--are sure to boost your chances.