Guild Outland Raider

I finished all the 25 man raids with my guild, it took a minimum of 8 people in the group to be enable us to earn the achievement.

As of 4.1, no longer requires Zul'Aman to complete since it will now be a Cataclysm instance.

Planning on faction transferring your guild?
Here's some Information for guild leaders concerning guild achievements based on single and meta-achievement guild runs.

If you partially complete a meta-achievement and then faction transfer your guild, there is a very real chance as of the current patch that you will render your guild meta-achievement "inactive". What will happen? You will be shown as having received credit for x achievement as part of the meta-achievement progress with a completion date in the tooltip but no assigned character. It will also appear greyed out even though the tooltip will show a completion day in the meta-achievement. The single achievement will show up normally. The only fix available currently is to put in a GM ticket and make sure you explain it carefully and provide screenshots.

Personal dealings: It took 10 tickets to get this issued resolved after 3 escalations on february 27, 2012. Why so many? My description of the issue actually. Every GM thought I was speaking of my personal achievements. So guild leaders, if this happened or happens to you, it can be fixed. Be patient with your GM's try to be very clear on the issue and add screenshots