High voltage, low current and vice versa
First of all, 30A in a short circuit wire with 12V is not safe. Chances are the thing will go bang or get extremely hot before you even have the chance to touch it. 

As for your actual question 'how is it possible to have high voltage and low current and low voltage and high current?'. That question you have sort of answered yourself. It's Ohm's Law. you have used the equations to work out the current in those batteries. Now, change the resistance in that circuit and you will see the voltage changes, hence that is how it is possible to have high voltage and low current, and also low voltage and high current. 

As for what damage can these currents do to a human.... Well, this link HERE will tell you all you need to know about fatal currents and electrical safety. There is also a discussion on what damage can be done in a question asked on this site HERE if you wanted a bit more of a read!