Imo. Healing proving grounds (and like healing in general )is about mana management. So I suggest you get the darkmoon chi ji trinket or go for the timeless trinket and reforge to spirit until you dont get oom in middle of fights.
Make sure you got glyph of totem recall and never use a HS or spam CL. try to make minimal use of GHW aswell and use talented UE on CD.