Remove any Greninja mentions such as the one you have in your checks and counters and the stuff mentioned in this reply but with a few changes.
Not QC, just my 2 cents.
- I'd definitely change the name of the set to "Physical Attacker" over underrated since its just more fitting, and such a trait is better described in the overview.
- "(and actually hits this generation)" remove this, its fluff.
- "and once Gyarados starts using Crunch+Waterfall Toxicroak is going to have a field day with it" dont mention this, as it is an XY analysis not and ORAS analysis.
- I feel that a lot of your checks and counters are a bit too specific, for example: "**Flying-type Priority**: Talonflame and Mega Pinsir easily KO Toxicroak with Brave Bird and Quick Attack respectively." This should be changed to Flying-types, and just mention that talonflame and mega pinsir, the two most common flying types, have priority to dispatch toxicroak. Staraptor and Skarmory are relevant too.
- "**fast Will-O-Wisp Mew**: Mew can burn Toxicroak whilst avoiding Sucker Punch with Softboiled, can Knock Off Toxicroak's item, and resists Drain Punch." This should just be changed to Mew, as they are always fast will o wisp variants.
- "**Faster mons with supereffective moves**: Landorus-Incarnate, Extrasensory Greninja, Staraptor and others can all take a Sucker Punch and OHKO Toxicroak." This should just be Faster Pokemon. Also mention that none of them can switch in easily.
Hope I helped.
The 1st point is fine, 2nd point as well. Don't even put Gyarados as something it handles well cause in practice you have to bank on the fact it hasn't been boosted yet and in the case of Gyarados you'll lose to Bounce and in the case of M-Gyarados you'll lose to EQ, which has enough relevancy to justify its use to make Toxicroak a very subpar check to this. Also the fact Mold Breaker Waterfall bypasses Dry Skin anyways so it doesn't beat M-Gyarados unless you've somehow put yourself in a good position against this. The 4th point is fine as well as it should be just Flying types in general with specifics being Talonflame, Skarmory, M-Pinsir primarily. 5th point should just be Mew and even then Mew doesn't like switching into Gunk Shot so emphasize this. Put all the 'faster mons in their respective categories such as Lando-T in ground types.
Checks to Ground-Types, such as Ferrothorn and Landorus-Therian, are very nice. Landorus can also take Specs Secret Sword from Keldeo quite nicely, allowing Toxicroak to pivot into Keldeo nicely.
These aren't exactly checks to ground types considering what the specific ground types generally run such as SD Garchomp, SD Gliscor, Earth Plate Lando-T, HP Ice Lando-I etc. Mention some partners in rain that break down ground types such as Kingdra and Omastar, with HP Ice Lando-I able to break past Lando-Ts and Gliscor along with physically defensive ground types who can't take the brunt of Lando-Is blows. Mention Tyranitar, Hydreigon, and M-Houndoom in team options for their ability to break down Psychic types cause some of your team options, not just for the sand portion, are really broad but don't touch upon specifics. Like you have more specifics for the aspect of rain but Toxicroak isn't only an asset in rain but also an asset for offensive teams to have a pseudo check to rain itself, a playstyle known to have a high advantage against offense which Toxicroak can slow down a bit. Basically anything that will appreciate its coverage options, utility in Dry Skin, and its ability to check both offensive and defensive Fairy types are what you want to convey for the use of Toxicroak on a team. With that said mention specific partners that fall under this category a bit more along with providing an asset to check some of the issues that Toxicroak needs out of the picture to successfully be used. If there are general checks and counters there should be some ideas of what can handle those under your team options. Also make a mention that it's an excellent Toxic Spikes absorber for team-mates that are primarily Toxic Spikes weak.
Mention M-Sableye in Checks and Counter for the necessity of having multiple boosts or it being worn down to beat this. Put Dragalge in there as well since you need Ice Punch to not get floored by a Draco Meteor. Put Garchomp under the ground type category for checks and counters as well.