Heroic: Ragnaros Guild Run

In order to obtain this achievment as a guild you need to have for 10-player: 8/10 people and for 25-player: 20/25.
Good Luck!

This achievement is currently bugged for guilds trying to earn it. From a Blizzard GM:

Thank you for contacting Blizzard Costumer Support!

As I can see you are still not able to receive the achievement "Heroic: Ragnaros Guild Run".

We have received multiple reports of this issue and can confirm it is not working as intended.
Currently, we do not have any workaround or resolution that can be provided.
We apologize for any inconvenience while our developers work as quickly as possible to resolve the issue.

Although already reported on our side, I also suggest you submit a bug report of your own. You see, player submitted bug reports are extremely important they are directly sent to our developers team with important information from your gameplay that can help the investigation reach better solutions for everyone.

https://support.blizzard.com/article/000172958

Bugs submitted in-game do not receive a personal response.
Instead, they'll be directed to the teams responsible for addressing them.

I hope I could helped you at least a bit further and I do wish you a grand day! :D
Best Regards,

☆ﾐ(o*･ω･)ﾉ