Heal Tempest
The Minstrel's Staff aura healer: 
Powerful Aura hands your auras to the squad and 
Elemental Bastion turns each one into a heal.
Shouts and water overloads do the sustained healing, Earth underneath is what lets you stand where the healing is needed. It is the heal build that asks you to hold still.
Why these traits
Powerful Aura is the hinge: every aura you give yourself goes to nearby allies, and Elemental Bastion then heals each ally you grant one to. 
Invigorating Torrents staples regeneration and vigor onto those same auras, 
Elemental Shielding adds protection, and 
Unstable Conduit makes each overload produce another aura to share — 
Soothing Ice and 
"Aftershock!" are the other sources. Earth pays for standing in the bomb: 
Rock Solid grants 
Stability to nearby allies every time you attune to earth, and 
Diamond Skin cleanses allies when you blast a field. The rest of the cleanse is 
Cleansing Wave on the water swap, which is one more reason the rotation keeps coming home to water.
Gear reasoning
Full Minstrel's — toughness, vitality, healing power and boon duration, and no damage at all. 
Monk adds more healing power and boon duration; 
Transference raises your healing to other allies by 10% and 
Life stacks healing power off kills. 
Karakosa heals nearby allies when you land a blast finisher, which is the same trigger Diamond Skin pays out on. 
Frenzy refunds two seconds of recharge on every equipped skill when something dies.
When not to run it
Overloads are channels, and 
Singularity makes you sit in an attunement before you even get one. Against a squad that hunts the backline you spend the fight cancelling casts and paying the recharge for nothing.
If that is the fight you expect, Heal Evoker puts the same job on meditations and a familiar instead. If the squad wants boons rather than raw healing, take Heal Catalyst.