What to write in application letter to address poor grades in Greek and Latin for classical archeology masters application? I am planning on applying to a Masters program in classical archaeology. 

BUT I have a B average in Greek and Latin combined. However, I feel confident in my abilities and that because I had to work harder to firmly understand the languages that I will be a better teacher and be able to relate better to my future students. 

What is a positive and effective way to address that in my letter?

Note: Unlike this question about overcoming a poor academic record in general, this question is about overcoming a specific deficiency in the introductory coursework that is considered essential for this area of study.

(The rest of my profile is as follows: I had one really bad semester when I transferred (all gen ed classes) and it totally screwed up my GPA. Since that bad semester I have built good professional relationships with faculty, done excellent work in classics, history and archaeology classes, presented several papers and did some independent studies (including one abroad on an archaeological field school). I have a clear vision of what I want to do for my MA thesis and beyond.)
As you stated a lot has changed since those unlucky days and you've achieved so many successes since then. IMO, write that you do not believe that the grade is a real indicator of your current status and as a proof of your claim you can reference your research and papers if it is available online.

Note: In case the professor of your archeology class would give you a recommendation letter it would be so valuable.