I'm aware of some of the meanings of とする but in this sentence
愕然 is apperently a noun that has something to do with feelings astonishment / shock etc
and one of the definitions of とする states that it has the meaning of to feel (e.g. after sound symbolism or psychological experience word)/to look/to feel like.
I don't quite understand the definition but i've just assumed that とする is used to express feelings but i can't find any examples other than with 愕然. In fact when i look for sentences with 愕然 they all have とする attached and i don't understand whether it's exclusively used with this noun or are there other examples.