I got the 'meek' and 'Infernal Ambassadors' acheivements last night.
I had to abandon and retake several times.
The trick to the Alpine Songbird part of the "And the Meek Shall Inherit Kalimdor" achievment is that neither YOU nor ANY-OTHER-BUGGER do any damage once Millagazor tries to escape.
The birds must finish him off.
Also, don't worry if you abandon and retake, or you notice that the birds disappear when you take the follow up quest.
Once he starts to escape the birds turn up all horror film like and kick his ass.
Job done. As Long as YOU STOP DPS and no-one else nearby kills him either!