An "achievement" guild would be a 25-man raiding guild working on hardmodes. That's where most of the achievement points are (since such a guild can easily work on 10-man achievements as well). There just aren't many achievements that require more than 1 player that aren't raiding or battleground achievements, and raid and BG achievements require dedicated skilled players (not all of them, but if you want to get a lot of points they do). Even being in a dedicated and successful 10-man raiding guild I am really struggling to find a path to get the 9000 point feat of strength that is coming, and that's with every Naxx 10-man and nearly every ulduar 10-man achievement and probably hundreds of hours invested into BGs.

If you are only going for easy achievements, or you are doing things with no particular skill standard, why would you need the support of a guild? Even just going for Glory of the Raider requires people with some skill and practice (or people who raid to overgear them). A nice friendly casual guild will help people get achievements, I'm sure, but I don't see the niche for a guild that focuses on them, since focusing on raiding and/or PvP is the way to get most of the points.