This achievement seems bugged in some arenas. I personally experienced this tonight: completed the requirements for the achievement 4 times in the ‘sylvari arena’, but got no credit for it. At first I thought I miscounted, but after the 4th time I started having my doubts… I ead the forums and moved to the ‘asuran arena’ and got the achievement there fairly quickly.
Additional testimonies 
: https://forum-en.gw2archive.eu/forum/livingworld/jubilee/Liadri-8-orb-achievement-bugged