It's essential that you craft some great equipment for the heroes, as that will increase their power and make battles a lot easier. There are various kinds of equipment, so depending upon the kind of hero you've got, use the right kind of equipment! For the very strong enemy heroes, use some ATK equipment, as that will significantly boost their damage versus monsters. Go either for DEF or speed if you have some therapists. The tanks will clearly require DEF or HP, depending upon their main stats, so read thoroughly each of your preferred hero's abilities, and prepare appropriately.