Don't know about Karachi but I have contact with my own sir, he might be able to help you out. He has also taught my friend (in a different school) who was world computer distinction holder. Maybe you guys could skype, call or whatever. But I don't think he would be able to come to karachi to get you geared up
Do read the stephen doyle book
if you want to get admission into govt colleges in Pakistan, then ace your normal 8 subjects all A*. A is a must, cant go with a B.4
However, if you intend to apply to other univ like LUMS/Aga Khan etc, then you must have some diversity i would say 10-12 is a good option. In my school 80% of students have 6A's and above in olevels in 2010 (so you can imagine that 8A's would be above average.