Here's where I address your questions about rules and post "how-to" videos to help further explain game play for newbies and new-comers as well as veterns alike
Is the AP column the only heading I can use to increase other action types? : Yes only AP may be used in this way. It also determines attacks per turn
Can I use my defences against a giant sized creature? : Yes, however to defend against a single giant sized attack you must use an modifier on your defence = to the giant's size. So a 3X size giant would require 3 Dodges to dodge against!