I have a 10 kHz PWM (0V (LOW) to 3V3 (HIGH)) which I wish to level shift as following; when PWM is high (3V3) the output should be 0V while when PWM is low (0V) the output should be -12V. Ideally this has to be done using discreate components (NPN/PNP transistors) while providing some sort of isolation (optical) between the 3V3 and -12V rails. In terms of current capabilities, the PWM pin can source/sink 8 to 12 mA max while I need not more than 200 mA for the -12V rail. Another requirement worth mentioning are the transient requirements, looking to have a clean 0 to -12V PWM signal.
Maybe you just need to use a decent high speed isolator with a buffered output like this: -
Available from virtually everyone including RS components. Of course, if you want to do it on the "el cheapo" you'll need a much lower load impedance than 10 kΩ and the addition of a buffer driver to give you 12 volt output compatibility. The RS part costs a couple of quid. I wouldn't waste my time trying to design something that gets anywhere near to the above ACPL-4800 performance.