Apple says to remove a case during charging if the iPhone gets too hot. The faster the iPhone charges, the hotter it gets during a charge, so try using the little 5 watt charger if you're not already.
On really hot days I place my iPhone on a frozen blue ice block wrapped in a cloth to keep it cool during charging.
Get a case that's easy to snap on & off. Don't use a case like the Spec candyshells or Otterbox. Those things can be extremely difficult to remove