First off, I appreciate the enthusiasm in learning grammar in your comments, but you don't need to explain your reasoning behind every change you do. You need to remember that these comments will be directed to the author, and not all authors will have the time or effort to improve their grammar and prose. In fact, explaining nearly everything you do can actually make it more difficult for authors to implement your changes. Try to limit your comments on changes that don't pertain to basic grammar rules or may need an explanation, and also try to keep comments as brief as possible.
To make things easier to point out for my feedback, I'll excise out some of your comments for readability and
highlight some parts of the paragraph for better clarity.
Slither-Wing Slither Wing is a niche defensive pivot which uses it's its interesting type, useful typing, (I think useful would fit better for describing Slither Wing's qualities in the metagame. A Normal or Fire/Ghost typing can be interesting to some people but they might not be useful types in the metagame in question) serviceable bulk, (AC) and reliable recovery in Morning Sun to check attackers like Great Tusk, Breloom, (AC) or and Kingambit.
- Everything after "which" is considered a essential phrase to the noun "pivot". For essential phrases, authors need to use "that", while nonessential phrases use "which" and a comma after the noun.
- While changing "type" to "typing" is correct here, we can't really change the author's words unless we have a good reason to do so. Your comment on changing "interesting" to "useful" is valid, but it'd be better to leave the comment and not make an alteration to the actual text.
U-Turn U-turn generates momentum for Slither-Wing's Slither Wing's team against mons Pokemon such as Flutter Mane and Skeledirge. (AP) and it And most importantly, (AC) it allows Slither-Wing Slither Wing to avoid being trapped by Shadow Tag (this sentence is ambiguous: if Gothitelle has Frisk or Competitive, Slither Wing can still switch out without U-turn) Gothitelle.
- You forgot to highlight "Slither Wing's" in blue.
- While splitting this sentence into two is a fine correction, adding a comma before "and" also suffices. Additionally, proper English grammar conventions generally don't have sentences start with "and".
- I understand elaborating on Gothitelle's ability, but I think given the context of competitive Pokemon and the type of readers that would most likely probably read this type of info, I think this is a bit unnecessary.
Close Combat OHKOs OHKOes Iron Bundle and Chi-Yu, (AC) and 2HKOs 2HKOes Miraidon, (AC) which Body Press cannot do.
- You forgot to highlight "OHKOes".
- Although it looks awkward grammatically, a comma isn't needed after Chi-Yu, since "2HKOes" is only part of a two-verb list with "OHKOes". Alternatively, adding "it" after the comma can work.
Will-o-Wisp's Burn Will-O-Wisp's burn stops the Pokemon Slither-Wing checks that check Slither Wing from using it as set up setup fodder. (RC, AP) it It also punishes opponents foes (opponent = opposing human player; foe = opposing Pokémon from the opponent) such as Toxapex or Corviknight for switching in.
The author is saying that Will-O-Wisp cripples Pokemon
that Slither Wing checks, not those that check Slither Wing. It is possible for a writer to omit the "that" for that phrasing.
Morning Sun heals Slither-Wing for 50% of its health, (Dex info) provides reliable recovery, helping it to consistently switch into Great Tusk.
- Good job on removing the dex info
- I think "to" can be kept here because that's a valid phrasing.
Tera Water provides a resistance to Fire type Fire-type attacks from Koraidon and Chi-Yu, (RC) and Water type Water-type attacks from Iron Bundle and Palafin allowing for a last ditch last-ditch counterattack.
- Good catch on "last-ditch"
- A comma is needed before "allowing", as it's a participial phrase of "Tera Water".
168 speed Speed EVs out speed adamant allow Slither Wing to outspeed Adamant Breloom and while the rest are dumped into (I think invested/allocated are more proper verb choices in here -- dump is a synonym for "disposing", "throwing away", and "getting rid of" (there's also dumping in computing, but it's not relevant in this context) -- I'd leave it to the original author to rephrase this part) physical bulk Defense.
- Good catch on the "outspeed" part
- A comma is needed after "Breloom", and you didn't need to change "and" to "while".
- Highlighting and commenting on a specific piece of text like what you've done here is exactly what you want to do if there are any questions or concerns over the author's writing; try to do so if there's anything similarly ambiguous like "interesting typing". That being said, "dumped into" is an adequately common and accepted phrasing for EV spread talk, so no need to worry about this for future reference.