I got the last missing Pict Boss =)
For me it did not give credit when I killed it. I did kill it three times in an Epic instance.
However - a GM confirmed only moments ago, that that particular boss should give credit towards the Pict Hunting Boss Achievment.
Pict:
Dengoro the Cannibal Connoisseur - Eiglophian Mountians - 793,1001
Cannibal placeholder