There is no Act called the Retrenchment Act. The provisions relating to retrenchment are available in the Industrial Disputes Act of 1947. According to the Industrial Disputes Act, managers (whether working in a manufacturing establishment, mine, plantation, or any other establishment) are not covered by any of the provisions of this Act. Therefore, the process of termination of a manager will be carried out as per the contract of employment/appointment order given and accepted by him. If there is a notice period mentioned in it, you can follow that.
A person designated as a manager need not be a manager so that the provisions of the Industrial Disputes Act shall not be made applicable to him. In order to be excluded from the scope of the Act, he should have managerial powers and responsibilities, like the power to appoint, authority to dismiss, authority to initiate disciplinary action, authority to appraise performance, or at least a power to approve leave.