Strengthen adverbs are provided in this article. The words listed here are commonly found along with the verb strengthen in sentences. This reference page helps answer the question what are some adverbs that describe or modify the verb STRENGTHEN.
actually, gradually
greatly, immensely
materially, merely
mutually, really
significantly, simply
ultimately
Hope this word list had the strengthen adverb you were looking for. In addition to adverbs used with strengthen, other example verb adverb pairings can be found on this site.
You can help by adding more adverbs in the comments. Thanks.