Short answer: You are, the client company. Using a professional employer organization (PEO) or staffing firm does not transfer the ACA employer mandate or the 1095-C reporting duty to the PEO. The client remains the common-law employer and the applicable large employer (ALE), though the PEO can offer coverage and file the forms on the client’s behalf.
For ACA purposes, the question is who the common-law employer is, and that is almost always the client business, not the PEO. So the client counts its own full-time employees to determine ALE status and bears responsibility for the employer shared-responsibility rules and for furnishing and filing Forms 1094-C and 1095-C.
A PEO can still help operationally. Under the rules, when a staffing firm is not the common law employer of the individual and the staffing firm makes an offer of coverage to the employee on behalf of the client employer, that offer is treated as the client’s own offer, as long as the client pays more for an employee who enrolls than for one who does not. The PEO can also prepare and file the 1095-C forms, but the client ALE remains legally responsible for the reporting and for any penalties.