The best place to hunt them in mass is in the Tunnels to the Harbinger (the Vincadi side) near Yanshi. Entering his dungeon requires handing a Harbinger Item to Vincadi in order to be flagged to enter the dungeon.
Speak to Zava bint Laruma again for the Slithis Slayer title and some experience.
Zava bint Laurma tells you, "So, you wish to prove your hunting prowess? Well then. Track down and slay 150 of those lurking Sishalti Tentacles, Tendrils and Eye Stalks for me and I will reward you appropriately."
Zava bint Laurma tells you, "Excellent work, friend! You have slain many of these creatures! Allow me to reward you!"
You've earned 23,106,533 experience. (max)
You have been given the title Slithis Slayer.
Zava bint Laurma tells you, "Ah, friend, it is good to see you back again! But we must take care not to wipe out all the Slithis, yes? Give them a couple of weeks to replenish their numbers, then come see me again!"
Notes
During the Reforging the Past event, the quest timer was standardized from 14 to 13 days.