This is a fairly straightforward repeatable quest. Speak to Guard Belton at the north entrance of Giran. He will tell you to gather Medusa's Posion, Wyrm's Tooth and Harpy's Feather so that he can create arrows to kill Antharas.
Go into Dragon Valley or, for lower levels, go towards Giran Castle. Between the road to Giran Castle and Dragon Valley is Gorgon's Flower Garden. Here you can find Medusas, Wyrms and Harpies. Hunt them and they will drop the relevant items.
Once you have your fill of items, bring them back to Guard Belton and he will reward you with adena. You don't need to have a full set; partials will do. Harpy Feather fetches around 97a per item. (To Do: Figure out how much the reward is for Medusa's Poison and Wyrm's Tooth.)
Drops of items are not guaranteed, and is about 50% for Harpies. Medusas and Wyrms have higher drops rates. Note that all three mobs are (reputedly) aggressive, although Wyrms seems more passive than the rest. Specifics on the mobs:
- Medusas have a Poison attack at Level 3, Intensity 6; this will take almost 1000 HP off, so make sure you have antidotes.
- Harpies have a Reduction in Movement spell which is annoying but hardly fatal (unless you are very careless).
- Wyrms have a nasty nasty Fireball spell that can knock off 500+ HP if you have poor magical defenses.