Benusta:
It's simple. You need to defeat Freya in the Ice Queen's Castle.
You can enter the Ice Queen's Castle in a party of 2-7 players of Lv. 115 or higher. You won't need to choose difficulty level here: Unlike daily instance zones, here there is only one difficulty.
If you defeat Freya, you will be able to re-enter the Ice Queen's Castle only after resetting each Wednesday at 06:30.
Note that during the battle you can re-enter the zone, but if all the members of your party left, you need to return there within 1 minute.