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: 
If you want to get it really right, then you need to say:

* To one woman: as-salāmu ʻalayki (or ʻalayk)
* To two women: as-salāmu ʻalaykumā
* To three or more women: as-salāmu ʻalaykunna (or ʻalaykunn)
* To a mixed group (men and women): as-salāmu ʻalaykum
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.