Answer:
outsource the accounting function to another firm
Explanation:
Based on this information regarding Marcus' situation, the best advice would be for him to outsource the accounting function to another firm. This is something that many individuals/companies do and will allow Marcus to focus all of his time and energy on what he is best at (which is helping his clients with their marketing challenges.) while at the same time making sure that the accounting tasks such as billing the clients are done quickly and correctly.