For mediation to work, all parties should involve persons empowered to make a decision to settle the matter. Make sure the opposition has a decisionmaker present at all times with authority to settle within the proposed range. If not, don't make concessions or settlement proposals, as you may be revealing your expectations prematurely. This lofty goal is not always achieved. Be Prepared to Make a Painful Business Decision.
Mediation is never easy.
An ultimatum may be delivered that may not be justifiable in terms of the law, principle, contract terms, dispassionate discussion, or industry practice. But it may make good business sense, despite the cost. At least examine the proposed settlement from that perspective. You don't have to make a deal. Just be open.