The fourth stage

The fourth stage is like the Ariant Coliseum, where you need to click on Elin to receive 10 "Transparent marbles" in the USE inventory to capture a monster.

Instead of scorpions, the monster to be captured is the Poison Sprite.
Like the Ariant Coliseum, weaken the Poison Sprite and than capture it (Use the Transparent Marble). The marble will then be converted/"purified" into an ETC item.
Once you successfully captured a Poisoned Spirte, the following animation will appear -


The party needs 20 of the ETC item. After collecting all 20, pass them to the leader and he will talk to Elin which will then "purify" one of the marble. An NPC "Sprite" will then appear. The leader needs to click on the NPC to go on to the next stage.
The stats of the Poison Sprite is as following -

Poisoned Sprite
Level: 65
HP: 7,500
MP: 100
Avoid: 0
Knockback: 1,200
EXP: -
If you accidentally kill one, it will automatically revive.