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!