"Best Method"

I would argue there's another easy method to train Healing, particularly in the later ranks that require one to heal someone in distress. It requires access to Scuabtuinne (through the completion of G23), particularly the Awakened Abyssal Lord mission.

1. Start the mission 2. Speed past all the turtles (preferably on a fast pet) and make sure they aren't chasing you, or they drop their aggro 3. Stand close to one of the switches with the red auras and wait until Deadly status is attained 4. Use Healing

When the gauge completely fills and the mission fails, the process can be repeated.

Do note that if you chose to go by yourself, Healing will consume Stamina... and a lot of it since you'll be doing this in succession. You may wish to bring lots of Stamina Potions. If Potion Poisoning is a concern, it would be very helpful to bring a Burning Flame Crystal which can be gained by contributing to the Campfire Spirit (via burning 10 items). The crystal will prevent the effect from occurring and it lasts 30 minutes. Otherwise, bring a friend along.