First i would say that you gem to much defence on your tank, 570 defence is a total waste, you could change atleast one of thoose defence gems to stamina or avoidance for better benefits.

Im not a priest expert but you should consider to respec holy for better healing! PoM,circle of healing and prayer of healing is the key to survive the elemental phase.

However the key to win this encounter is nature resistance totem and if you had taken a look on my movie you can see that im not even moving my slaves in the elemental phase. However i see you suffer from having slaves with very low health and poor spell damage but your tank outgear mine by far when i did this encounter!

But the key is nature resistance totem!

Remember that they both reset aggro when they emerge, i usualy taunt since you cant rely on avengers shield due to the long cooldown.

Dont use fire elementals use EARTH elementals! They are immune against nature damage and wont get splatted by the pukes of ooze he spits out.. Im not sure if they eat it but the earth elementals will certainly take some pressure of your tank!