For me, levelling up at mideel is the best one. Sometimes you might encounter 4 headhunters which 320AP which is nearly same as northern crater. (I don't want to train my materia there, because I afraid of L4 death cast by A flying creatures there, same as master tonberry). I just train at mideel until I master all my materia include KotR. By the way, did someone know how to beat emerald and ruby without losing any party member (like ruby) also can I get master materia more than two.