Enemies don't really have to act the way the player would act in order to create a tactical challenge for the player. They just have to act in a way that the player can counter. Your goal should be to create battles with specific player tactics, not with specific enemy tactics. After all, it's not the enemies that need to have fun playing the game.
If the player's optimal tactical counter for 75% of enemy strategies is to use the same skill over and over, that's when you have a problem. Most often, that skill is your normal attack, but I've played games where you just cast Firestorm or Spincut or whatever every single round, and it's not any more fun. You have to heal MP every ten or fifteen battles, and the spell animation takes longer, but otherwise the battles are still just as boring as Suikoden battles.