I think you meant, "How are you doing?" In that case, I'm doing wonderful.

"Who are you doing?" can be interpreted as an inquiry as with whom one is sleeping with. (That's a sexual connotation here in the US.)

I think he want to know how are we doing...? 

for me am doing fine dude what about ya?

what are you doing??

i am having brunch with my mum.

What are you doing , which I suppose that is what you want to say

I am writing in English to respond to your question .

?????

How are you doing???  I'm doing good

What are you doing?? I'm working while reviewing my Chinese lesson