If you want to write a congratulations letter to an employee for an achievement, here is a simple template you can use to write an achievement letter for an employee.
You have achieved a great and almost impossible feat and have proven the skeptics wrong.
I am certain that you and this achievement will be in the public eye in the industry. You will definitely be admired for this feat within our company and in the public.
Offering you my sincere congratulations for this milestone.