****UPDATE 30/08/2018 - Had word from the original creator of this venture that it was cleared out and is no longer available. As per below in the comments, try searching for suggested keywords to try and catch when someone else is running or close by to finishing a 500 level venture. Good while it lasted.
***UPDATE - This solution still works as per cbenson comment below. Venture is set to infinite time so you are able to join, participate in any way you can (be that just dying instantly) and you will be rewarded - May require game exit and re-launching.
Not so much a guide as a heads up really....will update once this map gets cleared and useless.
Just to make you aware that a community level by a Discord moderator is getting close to surpassing level 500. (Currently it's TOP 417 so not that long to go)
Access menu with the floating Islands 'The Spiral Shards' which appears after selecting a character.
Press RT 'Right Trigger'
Search by creator - HordeofTribbles
Map should appear called 'Rare x8 Please incapacitate boss until 500'
Now play that map. Doesn't matter if you die as long as you take part.. If you do manage to beat a boss then please incapacitate it to carry on the venture all the way to 500.
Now.. eventually this map will get beyond 500 and when it does and is finished. You should get a notification when you next log in, or back to spiral shards screen where you can pick up the reward and the achievement will pop.
You should be able to pick up the registered streamer achievement on this one too if you haven't already.