Well, in case you haven't already gotten past Barius Hill......the most obvious solution would be to get into some random battles and level everyone up. And just focus on getting rid of those summoners. Also, it would be a good idea to keep your party spaced out so that they all don't get hit with a Ramuh attack or anything. But one of my favorite things to do is to cast Silf on them so they're silenced and therefore completely useless.