Cassandra was a natural fit for the role because of her well-refined combat skills.
In the above quote (from a piece I wrote for my job), I have used the prepositional phrase 'because of her well-refined combat skills'. Ordinarily, I would categorise this prepositional phrase as an adjunct, modifying the main verb of the clause (i.e. 'was'), but I am curious whether this is true for copular/linking verbs. It seems quite existential, modifying a state of being rather than an action.