A skill for healers

Depends on a lot of things. I'm assuming they're at least getting Hone Cavalry buffs. Brazen Atk/Spd pairs well, especially with Miracle. Fear is a great staff, but it's all really about their role in the group. A Pain Savage build wants Vantage and Close Counter.

You could try using Bond skills, but that requires careful positioning.

You forgot bond skills. I find bonds difficult to use on cavalry because they like to romp around rather than stick together, though, so if you're using a healer in a cavalry team I cannot recommend a bond skill unless your tactics are quite strong.

I would say it depends on what your plan is with the healer and their IVs. For example, if you dump their def, brazen atk/def doesn't make much sense, but if you invest in their def, then it might.

The challenge with brazen skills, especially for a healer, is activating them. They're not good at taking physical damage, especially if you focus on their offensive potential, and there's no guarantee the enemy will provide a mage you can tank. (Reinhardt probably hits too hard, and a lot of mages are too fast for even those three to dodge a double without hone cavalry, or sometimes even with.)

Also, the question of what your offensive plan is. Trying to tank with absorb+ is different from splashing everybody with pain+ double savage blow is different from dulling their offense with fear+ or slow+.