Save the game and set fire to the corpse dragged from the well. After that, the Devil by the Well will appear. It's a noonwraith. During the battle, avoid stronger attacks of the monster. Use Yrden sign to create traps and weaken the noonwraith. You could also use some bombs called moon dust but that would require gathering some ingredients and producing them before the battle. Be watchful of your health bar, heal yourself whenever it is needed.
After defeating the wraith (50 experience points), examine its corpse and gain noonwraith trophy. You can return to Odolan and tell him you have cleansed the village (50 experience points). During the meeting you can receive your reward (the amount you previously bargained) or let the man keep his gold. If you choose the second option, you will receive an amethyst.
Main reward for completing witcher contract: Noonwraith trophy - 5% additional experience for final blow in battle with humans.