" The main party are dead! And the reserves are level 14! Im fighting Yiazmat! Omega Mark XII! Gilgamesh! the undying! OMG we are going to die!"

You will face that situation if negeclet people. I suggest making reserves about 5 levels behind your main. But thier levels is up to you. OR You could make everyone the same level. I suggest everyone same level. More safer.