use "Unlock" skill that daggers have and open the Secret Walls. You will need at least a level 40 dagger and 5 Thief Keys for each door. Be careful that sometimes the skill will fail (so bring many keys with you) and after a while the doors will close so you will have to open them again.
The raid bosses alive in this video: Eva's Spirit Niniel, Fafurion's Envoy Pingolpin and The 3rd Underwater Guardian. Unfortunately, Eva's Guardian Milenu was dead when I've recorded the video.
Lineage II Official Server