Nobody can really tell you how difficult the test is because it's about 90% memory and 10% computation, and even those questions involve remembering the formulas.
If you can recall the information, it'll be a breeze, if not, well you'll be sitting there scratching your head for a while and probably not do very well.
Specific things that you need to know for the test :
-Ohms law!!!!!!!!!!!!!!!!!! (calculating wattage, amperage, voltage, and resistance)
-Every question from the practice test is on the real test
-Resistor bands
-Crossover components and their functions (Inductors, capacitors, etc)
-How to wire a relay
-Basic electrical components and their functions (Diodes, resistors, transistors, etc)
-Know proper wire routing
-There is a surprising amount of shop operation/safety questions (Protecting client vehicles, fire extinguishers, crap like that)
There's plenty of other stuff but that's all the really specific things I can tell you.