The AFTA assessment day is very easy compared to cadetships like Aer lingus, City Jet, Ryanair etc. If you do a small bit of study you will be fine, no need to worry 1 month out. They will assess basic math, physics and logical reasoning. They also have a small flying portion which is easy enough. The only bit that could be considered challenging is the interview, as long as you can show you like aviation and know a few bits you will be fine.