It depends on many factors, like the level & magic level of the druid, test the amount the exura sio heals or calculate it (tibia-stats.com have a great calculator), but around level 400 would be just fine, just make sure to wear a good physical protection set, as you will not be wearing a shield, just make sure to not wear anything that lowers your holy, earth and energy damage resistance, as they hit hard with these elements. Also make sure to wear a Prismatic Ring every time, in case things get rough swap to an energy ring just until your HP is healed again.
Hope I've helped ^^