I do note that one 85 guild member soloing a level 10 dungeon for guild credit does meet the spirit of the achievement, but there is another side. Recently my guild completed Naxx 10 man raid with 6 guild members. 5 of the 6 were 80, and we had one 81. We did not get the achievement because it requires 8/10. This seems to meet the spirit of the achievement.
I would suggest a % requirement on guild achievements.
Dungeons (80% Guild of level appropriate guild members or maybe slightly larger level range)
Raids (80% Guild) - It is probably unrealistic to ever have your guild finish 60 raid content with level appropriate toons.
If my guild can beat level appropriate content with less than the required number of guild members, we should get credit and guild experience.