This is the case I have in mind. I wish to express that impact acted in a way that was severely adverse.
It impacted her severely adversely.
The proposed text above doesn't feel right at all, though.
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Not all adverbs are allowed to modify other adverbs. The adverb severely cannot modify another adverb.
You could correct the sentence by saying "extremely severely" or by saying "it had a severely adverse impact on her".
How do you determine which adverbs can modify other adverbs (or adjectives or verbs, for that matter)? I don't know; I haven't found any lists online, and none of the dictionaries I've checked classify adverbs according to what they are able to modify.