Newbie Helper:
Go and get a blue gem! Don't you remember? You've got to kill gremlins!
As I said, move your cursor over a gremlin and click the left mouse button. This will transform the cursor into the shape of a knife. Click the left mouse button again to attack.
Use Soul Cry magic before attacking. This will improve your chances greatly. To activate Soul Cry magic, press F2 key, and again to de-activate it.

Do this and you shall be well-rewarded. Good luck!