Can be found in Ambassador Wing's Dungeon
When her hp drops 35%, she will have Resistant to Death effect (invulnerable). To deal damage under this state, you have to die once and become undead (getting healed will do damages). This can be achieved by simply dying the normal way or end your turn on the death tile. You will be revived the next 2 or 3 turns before knocked out.
Do not let all party members die at once, it will make the fight over. Leave at least one party member under the state "alive"