Question: Normally we say "Assalam o Alaikum", but my sir said that for women we need to say "Assalam o Alaikuna". Is that right?

Answer: 
Even **"Assalam o Alaikum"** could be used for women, it is more convenient to use **"Assalam o Alaikun"** ( without the appended -a ); unless you want to say another thing else without stopping, then you can use **"Assalam o Alikuna** (oh mothers/sisters or something else)". I wish it helps.
As much as I see, it can use for both men and women in informal form. But if you are looking for the formal form of the greeting or saying salam for women, you should be aware that if there are more than 2 women, you should say Assalamo Alaikon.

There is another important matter that you can note, and that is about two women. In fact if there are two women, you should say Assalamo alaikoma in the form of formal and in fact according to the true grammar of Arabic.