How to face the unforeseen implantation of a new competitor in the sector,(a renowned and commercially aggressive company)?

Know your competitor. What drives and motivates them? What are they doing differently? Know the answer to "why". Why do you do business why do you create what you create make sure your clients know the reasons why and market this. If you have a passion or dream let your clients know about it let them feel good about your dream too it will attract like minded clients and they are the ones you will keep.