(This guide is based on eAthena, in official server you need at least job level 40 to start the quest)
Talk to him, he will ask you to bring a report back from a soldier named Sloutii located at south of Geffen.
Shrink - Step 2
Soldier (Sloutii) gef_fild13,297,242
From the west gate of Geffen town. Go 1 map west and then 1 map south. (It's the Geffen Guild Base map.) He is standing outside of the castle at the top right.
Talk to him, give him the items he asks for and let him make the antidote.
Talk to him again to get the newly made antidote (you will not get the physical item).
Now, you have to go bring the antidote back to Sloutii. There is only a 1/5 chance that the antidote can cure him. If it fails, go back to Father Arthur to get another antidote (you will NOT need to collect the items again).
Go back and forth until you can cure Sloutii and he will teach you the skill.