Conni
Dugtrio is a great revenge-killer (hyphen shouldn't be used here) that is also able to clean late game due to it's high Sspeed.
-good call on removing the hyphen;
-it's -> its here because it's a possessive pronoun, and late game -> late-game because that phrase is hyphenated;
-i would've kept "Speed" here because you're primarily referencing the Speed stat.
Make sure to scout for opposing Scarfers pokemon with choice scarfs (Scarfers isn't really spelled correctly and has grammar issues), (no need for comma since because is already a sentence expander) because they can outspeed and KO take out Dugtrio instead.
-Scarfers is too casual and therefore doesn't fly yeah, your fix works with the words capitalised that need capitalising, but something like "Choice Scarf users" would be a lot less clunky;
-"okay" on removing the comma before because, don't think a comma is entirely wrong there but i'd say it does work better without one so w/e;
-no real reason to change KO to take out.
Because (don't start a sentence with because) Dugtrio it is so frail , Dugtrio because (better to use because) it is unable to switch-in on many attacks,
-switch-in -> switch in. switch in is the verb form, switch-in refers to a pokemon switching into a foe;
-there is absolutely nothing wrong with starting a sentence with because; that only applies to coordinating conjunctions (and / but / or / yet / so / couple that i forget, and only in formal writing) for reasons that are a little too technical to elaborate on right now, "because" and other subordinating conjunctions are fine;
-be careful with causality here, because your changes have kind of put it the wrong way around. dugtrio's frailty is the reason it cannot switch in on attacks, its inability to switch in is not the reason it's frail. the original sentence was fine here honestly apart from the hyphen in switch-in.
so you should only bring in it Dugtrio in after a team(remove space)mate has fainted or with when a slow Volt Switch/U-Turn/Baton Pass user such as mMega Ampharos and Vaporeon.
-good spelling fixes;
-U-
turn, not U-
Turn;
-"only bring it in" wouldve been fine imo, it was clear enough that Dugtrio was being referred to;
-"with" was fine too;
-those slashes are kinda too casual for an analysis, "Volt Switch, U-turn, or Baton Pass user" would've been the way to go.
It's best to set(add space here)up DD towards the end of the game, when Mega Charizard-X's(don't put a hyphen between Charizard/Mewtwo and X/Y) checks and counters are out of play or have been weakened enough to be picked off defeated (better to use defeated as it is understood easily) after a boost.
-good call on adding the space;
-spell out Dragon Dance entirely;
-good forme name fix;
-i don't like the change picked off -> defeated because there was never a real reason to ("pick off" is understood easily enough) and "defeated" gets rid of the nuance that its counters have to be worn down first, which is kind of important and "picked off" conveys fine.
Charizard-X (don't put a hyphen between Charizard/Mewtwo and X/Y) should set up on a Pokemon that it forces out, such as Mega Scizor and Celebi, or against Electric-types such as Manectric-Mega Mega Manectric and Raikou.
-good fixes on the forme names, though don't forget to add "Mega" before charizard x as well;
-be careful with removing that comma after celebi, "such as Mega Scizor or Celebi" should be bracketed off either by two commas or by none at all;
-kinda technical, but since "a Pokemon" is singular, make it "Mega Scizor
or Celebi", because the example has to be a singular too. on the other hand, "and" is correct with "Mega Manectric and Raikou" though, because "Electric-types" is plural.
Rotom-Wash is an easy target to set-up on if it is running HP investment, as it can't 2HKO Charizard-X with hHydro pPump, and is stalled out with Roost.
-rotom-wash -> rotom-w, truncated forme names are the way to go. same with thundurus-t, giratina-o, wormadam-g, etc;
-set-up -> set up. set up is the verb form, setup is the noun, and set-up is an adjective / participle ("a fully set-up scizor" is p. much the only correct way to use it);
-good call on removing the hyphen again, but remember to make it
mega charizard x as well;
-no comma before "and"; if you have a comma there, you should also introduce a new subject after the comma. "i played football and kicked him" is fine, as is "i played football, and i kicked him", but "i played football, and kicked him" won't fly. kind of hard to explain properly, but it helps if you view it as "what did i do?" "play football, and kick him"; this makes about as much sense as "heatran, and scizor are good fairy checks", so just get rid of that comma there.
Make sure you avoid Thunder Wave from pokemon like Cresselia, Thundurus (which also has Prankster) and Mega-Slowbro as paralys paralysis cuts Mega Charizard-X's speed in half, making it suspectible susceptible to a higher amount of revengers pokemon.
-capitalise pokemon and speed;
-good call on removing hyphens and fixing misspellings;
-it quarters Speed, it doesn't halve it. tbqf "cuts Mega Charizard X's Speed" is probably good enough too, no need to get overly specific if it makes the text too clunky;
-always put a comma before as / since if they mean "because";
-revengers doesn't work yeah, but "revenge killers" is a more standard phrase;
-kinda technical, but don't use "amount" for things that you can count, use "number" instead. "the amount of sand" vs. "the number of grains".
this probably looks like a lot to take in at once, and i don't expect you to get it right immediately anyway haha. overall i'd say you know what things to be on the lookout for, and i'd like to see more after you familiarise yourself a little more with our grammar standards / become a little more consistent :)