If you have to write a letter of recommendation for a fired employee, here are a few suggested phrases.
For a chronically absent employee: A man like him is hard to find.
For the office drunk: Every hour with him was a happy hour.
For a chronically absent employee: It seemed her career was just taking off.
For a dishonest employee: Her true ability was deceiving.
For an employee who is so unproductive that the job is better left unfilled: I can assure you that no person would be better for the job.
Another fun linguistics way of subtly insulting someone is “damning with faint praise”, which violates the Gricean maxim of relevance in order to give the impression that there aren’t any more relevant, positive characteristics of the person. For example, if someone says “how did you like the movie?” and you say “well, there were actors and costumes and I think it was even in English”, you’re probably implying that it wasn’t very good.