That woman didn't say what it looked like. She only mentioned that they would be in the castle.

[Location isn't a bad start, but they needed to know what they were hunting. Luckily, Gareki isn't dumb enough to charge into anything without doing a little bit of research.]

But what we looking for are eyes. I happened to look it up before coming here. It's a blob-like creature with multiple eyes. Killing just one should be more than enough... Unless of course you end up beating it into a pulp with nothing good to take.