How to explain your manager that he/she is having the wrong strategy for a specific goal without hurting him or her?

Find the time and opportunity to prepare the comparison scenarios for both strategies: your manager's and the one you feel is right. If required, prepare the calculations and effectiveness analysis. Short presentation (10-15 minutes) once your boss agreed to discuss with you, most of the managers are not ready to receive the critical remarks, but if your presentation is of the constructive type, and you prepared the smart proposal to improve the strategic approach, it will work for both- your manager & your company.