Thanks for the answer 
Since I'm KE with special armour that gives extra Guard everytime I got hit, it's quite easy to keep the life full all the time since the barrier got recharged everytime I hit the dragon with Spirit Blade. My tactic is to hit it with Spirit Blade since the damage is quite good compared to staff attack, especially when the dragon start to flap its wing to do area damage. If I can hitting at that time, it's a guarantee to keep the barrier up all the time.
The problem is dragonling. There are more and more the lower the dragon's life point drop. Everytime it takes me more time to kill all & they give less guard & barrier recharge.
Your remark about fire resis/regen potion is really good, I'll try it. This potions I bring on that screenshot are the potions/grenade I always bring. Normally the Healing Mist is to restore the HP of the team. Guess that next time I'll also respec to ice mage first.
Does mana pot affect Spirit Blade?
Edit: other problem is the time needed for this battle. It took me 2h straight & I got more & more tired so I started to make mistakes & got my healing pot depleted.
I don't know whether this is feasible...