"I think you're ready for more of a challenge. Head north from Laksy's Anchor. There will be sagittariuses. Slay some of these to practice your skills. But you'd best be cautious, they are much stronger than anything you've faced up to this point."
After accepting the quest, teleport down to Laksy's Anchor and head north from there. The Sagittarius can be found just over the bridge leading north from the Laksy's Anchor. Kill the required number of monsters and then return to Guide Prometh.
"Awesome. If you can take out Sagittariuses, you need not be afraid of most monsters on the continent. Of course, I am only saying that as encouragement. There are many creatures out there that could take you out in one blow."