Linkshells.

I put them in the Hunt LS (even though these are fates).

Something like "Achievement Fate up at <pos>. Need help" (mentioning that it's for an achievement is why I've yet to find anyone that minds that a HUNT LS has a fate relayed in it).
People usually come as long as it's no a bad time for the server (like 3 AM on a NA server isn't a good time to be asking for help with achievement fates, but 8PM's usually good).