Excluding Pokemon that can do infinite damage such as Sizzlipede's Relentless Flames, M Mewtwo EX's Psychic Infinity, or Mr. Mime GX's Breakdown, here are some Pokemon that can do 300 damage or more.
If you put 8 water energy in Slowpoke & Psyduck GX's GX attack Thrilling Times, you have a 1 in 1024 chance of dealing 1010 damage. https://assets.pokemon.com/assets/cms2/img/cards/web/SM11/SM11_EN_35.png
M Tyranitar EX can do way more than 300 damage with Destroyer King but I don't know the max (I don't want to do the calculations) https://assets.pokemon.com/assets/cms2/img/cards/web/XY7/XY7_EN_43.png
M Rayquaza EX, M Charizard EX, and Charizard GX can all do 300 damage.
And if you use Slowbro's Three Strikes, you have a 1 in 8 chance of dealing 300 damage.
I also found that Reshiram & Charizard can do 300 damage with it's Double Blaze GX attack.
And if you use Empoleon's Total Command and both players use Sky Field and have 8 benched Pokemon, that means there's gonna be 16 benched Pokemon and you're gonna do a total of 320 damage.
And just like M Tyranitar EX, Sableye V's Crazy Claws can do more than 300 damage.
And here is another version of M Charizard EX that can do 300 damage:
And also Silvally can do 400 damage if your opponent uses Sky Field.
Chandelure can also do more than 300 damage with it's Vortex of Pain.
And Shadow Lugia can do 1000 damage, but that has been banned from legal tournaments.
And I didn't include weaknesses because if I did then the min damage required is 150 and there would be much more Pokemon on this list.
And note that I also didn't include Trainers that directly increase damage done by Pokemon like Vitality Band or Martial Arts Dojo and attacks that depend on the damage dealt by your opponent like Cinderace Vmax's Counter, Wobuffet V's Gritty Comeback, Spiritomb's Anguish Cry, or Alolan Ninetales GX's Ice Path.
And I also didn't include Pinsir's Guillotine Hug, Absol's Doom News, Umbreon & Darkrai GX's Dark Moon, Trevenant & Dusknoir GX's Pale Moon, Alolan Ninetales GX's Sublimination, and Darkrai GX's Dead End, and Silvally GX's Silver Knight (Knocking out a Pokemon is sort of the equivalent of dealing the amount of damage equal to that Pokemon's hp).
Neither did I include Pokemon like Moltres, Zapdos, and Articuno GX whose GX attack Sky Legends can deal a total of 330 damage nor Reshiram & Zekrom GX whose GX attack Cross Break can deal a total of 340 damage, nor Magikarp & Wailord GX, whose GX attack can deal a total of 810 damage if your opponent uses Sky Field, nor did I include Lycanroc GX whose GX attack can do a total of 400 damage if your opponent uses Sky Field, again nor did I include Horsea GX, again whose GX attack can deal a total of 360 damage also if your opponent uses Sky Field (it's a useful trainer).
And to answer the hp question, the Pokemon with the highest hp is currently Snorlax Vmax with 340 hp, and Buff Padding raises it to a whopping 390 hp. https://assets.pokemon.com/assets/cms2/img/cards/web/SWSH1/SWSH1_EN_142.png
I forgot to include Hitmontop which can do a lot of damage with Finishing Combo.
And also M Blaziken EX's Moonsault Blaze can do an infinite amount of damage.
And M Heracross EX can do infinite damage if there was such thing as a negative damage counter :P
And M Slowbro EX is sort of like M Blaziken EX.