.. Getting some fish oil

You spotted the oil extractor near the cottage? Just get a fish, first.

Put the fish into the oil extractor and use the extractor the get the fish oil.

Oh, right. Who would give you that?

Talk to the boy in the room. Ask about the fish oil and a special container.

Use it on the extractor. Now you get the flask full of fish oil.

Give it to the boy. You get an elf in a jar that will attack anyone close to it.