I'm working on the longbow of moon questline but ran into a problem twice now.

When going to heine 4f finding the cursed exorcist saell and using the blessed water to be able to attack it, I cannot seem to kill it. I have been trying for several minutes the first time. Second time I did not stop attacking until the timer on the blessed water ran out.

Any ideas on what im doing wrong? Is it bugged in some way? 

https://l1justice.com/lineage/database/monsters/45941/

it has 500hp regen, need to nuke it down (within 4 or 5secs). it has only 800hp.

should be easy as int elf with thebes bow.