1. travel to sub-mission Shadow of the Sacred Mountain (part 3)
2. jump up on the rock pillars to your right and grab the dragon's cure powder
3. jump down to the flag and travel to the same location
4. pick up another powder and keep doing until you have 10 of them - if you have fast loading times it should take you just a couple of minutes

now you have 10 surprisingly powerful regen items that should boost your healing capabilities by a lot. Equip them to your quick slot and use one and start face tanking bosses. Repeat until all bosses are dead.