If you're like me and want to 100% the achievements last second I have the solution for you.
For some reason the devs thought it would be funny to make an achievement take 500 matches in a party when the next closest one would be 300 wins. But there is easy cheese to doing it quickly.

First off you are going to pick a character you do not use at all. Every time you win a match you get 1 point added to a badge that lists your wins. Since the devs thought it would be funny to not be able to track the progress this is the next best thing to keep count.
Second find someone insane enough to do this with you. You have until May 30th so there is still a little bit of time left.
Third, once you have that set up go into rifts, it counts towards your progress I got the achievement from there. Go to Season 1 and choose Multiversal Madness. From there keep playing until you get to "Break the Targets".
Fourth, we start grinding. Jump off into the blast zone over and over again. I checked most of the rifts until the 6th one and this one is the fastest one so far. On average if you're starting the achievement it should take 4 hoursish to do this 500 times. Equip the wins badge for the character you're using so that you can keep track of how many times you've held left and died.
Fifth, BOGO Deal. If you have not done the 300 wins achievement "The Throne is Yours" you will get it while doing Party Animal.
You do not have to do it all at once, it gets very mind numbing to do this over and over again. I used to play Runescape so I'm used to doing something over and over again to see a number rise up. You'll soon see that your worst enemy is going to be the load times. You have to pick the stage every single time you want to do it again which is funny because in singleplayer there's a retry option but I digress. I wish you all good luck in holding the left position over and over again. Also if I put the text in the wrong box tell me so I can fix it lol