Your query is devoid of information. Is the manpower mentioned workmen, staff, or executives? If the surplus manpower is from one department, how was it declared, and based on what study? This information is required for clarity and consistency, and to reduce potential litigation.
What happened to other departments, and has a similar study been made for them?
Think of the possibility of job rotation or redeployment in other departments where less manpower is declared or could be accommodated, or to branches or sister units of the same management, without any loss in service continuity and wages/pay.
Retrenchment should be the final option. It should be based on the overall strength of the organization and follow the "Last come, first to go" principle. You may refer to Chapter V-A and V-B of the Industrial Disputes Act.