depending on raid composition you should focus 2 or three fingers at the time.
When you damage a finger its starts to countdown to twenty before stacking debuff on its target.
We always assign range DPS to help melee to nuke it then clear middle ones that healers stand at and then kill ranged ones.
When we were progressing this fight, we had issue with tanks not killing orbs with hand's slams and people not getting out of purple puddles.
Once your tanks do their job and people move out of purple then healing is not as difficult and you will be fine.
Thank you for tips, she went down a week ago (or so). We have Jugg+PT tanks, Sorc+Merc healer, Jugg, Mara, Sniper, Merc DPS. However, the healing in this phase was really intensive.
It happens that a healer gets 2 - 3 purple aoe under theit feet in a row resulting in death :-) Generally, it seems that the purple aoe bombs are killing us most preventing good positioning to a finger or being hardly visible under sorc bubbles etc.
Anyway, she is down and will go down again :-)