Hi tazeri, and welcome to the forums.
It's not clear if X actually created the advertising campagns, wrote the leaflets and brochures, etc., or if she coordinated their delivery. It's a big difference, but I can't tell from your description.
To whom it may concern: There is a lot of discussion about the use of this phrase. Grammatically it's correct, but some advocate for Dear Sir or Madam:
X.Y. has been with our company's marketing department for a year, working as a paid intern as a Brand Manager trainee, under my direct management. I enthusiastically recommend her to postgraduate Mass Communications programme at the University of Leicester.
[Company name] is one of the largest producers of medical equipment and hygiene products in the Czech Republic. X has worked on advertising campaigns and the creation of marketing materials, including leaflets and product brochures. Her skills and understanding of our business was such that I placed her in charge of developing some of our new product launch packets. She has repeatedly demonstrated both her creativity and her sense of responsiblity. Her studies have enabled her to offer very useful tips and on-point recommendations concerning our media activities. As just one example, her rating analysis of radio stations helped us in our selection of the right station and advertising time slot to reach our target audiences.
X has put into practice the theory she had learned about research methods, preparing surveys and questionnaires for our customer briefs. I'm not sure what a customer brief is. Her most recently research project was our regional "mystery shopper," focusing on pharmacies. X created the questionnaires for our mystery shopping agents and analysis of the data obtained, and also provided administrative support for the whole project.
I'm confident that X’s tenure at our company has been extremely beneficial to both parties. She is a valued asset to our company. We will be sorry to see her move on and continue her studies. I'm also confident in her future success, both in business and academically in your programme.
If you have any questions concerning my recommendation or require further information on X’s work performance, please don’t hesitate to contact me.
Best regards, There has also been a lot of comment about closings in letters. As an American, I use "sincerely" for everything, but I think that "Yours truly," is correct in this case. You'll want to confirm that though!
Ing. A.B.
X is lucky that she had a great manager in her internship and should be very happy with this letter.