I just completed my first set (got my results today) and Gen Nav was the one I messed up. I don't find it too hard but the paper we had was mind boggling! All I can say is when it comes to the exam, time is your worst enemy so get used to the time constraints involved. Gen nav is sometimes hard to digest but everything links hand in hand logicaly, but if you don't understand one thing it tends to have a knock on effect!
Good luck