I forgot to include the following sentences, which I really hope you can check, too.

1) The woman is horrified by (or at?) looking at her image reflected in the mirror because she can't accept growing old. (is the gerund correct?)
2)The mirror can be regarded as a symbol of truth, in contrast to the moon and the sun, which do not project a faithful image of the woman.
3)The Celts established themselves/settled (both correct?) in Great Britain in the 8th century.There were good iron workers (instead of "good at working iron") and based their economy on hunting and fishing.
4) He makes use of (uses) a metaphor when he refers to human beings as dingy shades.
5)A third-person unobtrusive narrator shows what happens without interfering in the story.
6) An anaphora is a figure of speech in which... (is "where" possible?)
7) A distinction should be made between flat and round characters. The former, also called types, do not develop in the story ("throughout", "during", "in the course of" the story are possible?, whereas the latter change their personality as the narration develops.
8) Your sentences are either confused or meaningless (or don't make sense at all?)

#1 horrified by is correct, and your gerund is fine.

#3. established themselves or settled - do not use themselves after settled.

Good iron workers is correct.

4. uses is better

6. which - not in which or where

7. do not develop - (all of those can be used) - the story

8. Any of those would be correct.