I don't think I can even come close to Kabooa's elaboracy, but my idea is mostly inspired from this entry:

https://www.d20pfsrd.com/classes/bas...es/gunslinger/
"Stop Bleeding: The gunslinger makes a firearm attack and then presses the hot barrel against herself or an adjacent creature to staunch a bleeding wound. Instead of dealing damage, the shot ends a single bleed condition affecting the creature."

A job originating from the need to have a combat medic, but with the lack of mastery of mana, a sect of soldiers found ways to staunch the wounds of themselves and their allies. Though definitely unrefined, this sect, branding themselves "Corpsmen" have found many ways to bandage and strap together allies in the midst of the battlefield with weapons of war.

The idea of the job would revolve around doing damage to heal.

But not necessarily the enemy.

Cauterizing a wound comes to mind: that entails burning a wound to stop the bleeding as I understand it. So the idea behind the healer would be they harm the tank, but in harming them they create shields that lower incoming damage. Perhaps a dot that ticks for 50 potency damage on the tank, then 100 potency for a heal immediately after.

Its weapon would be a gun.

This is probably a really bad idea for a healer, and very counter intuitive. But it's something that has been rattling about in my head for a bit.