There is no defense against such attacks other than "magic". If you try to take this by science, you'll fail.
The reason is simple:
a) Lightning doesn't work in such a way as it goes one particular way as pointed by the wizard, and damages the target. Lightning is electricity, so there must be a closed circuit. Current is everywhere within that circuit. Fullstop.
Lightning fired from a wizard's fingertip would damage the wizard as much (or more!) as the target. The current would go through the ionized air, then wander over the Faraday cage surrounding the target (metal armour) without causing significant damage, and close the circuit via the ground, thereby frying the wizard.
b) Magical fire (burning hands, dragon breath?) will hard boil anyone inside a piece of armor whether the armor stops the fire or not. Plus, it will render air unbreathable (both due to lack of oxygen and temperature). Metal has high thermal conductivity, but even with thick layers of insulation (try and move with one foot thick of bandages wrapped around your thighs), the temperature inside would rise beyond what's tolerable for a human anyway. Even with modern technology and with appropriate breathing equipment, standing in a definitively non-magical fire is (although tolerable for a while) already not a big pleasure. Add to that the fact that usually magical fire is thought to burn "extra hot" so the metal would melt anyway.
The same as with lightning applies for magical fire in respect of the wizard, too. Again, it works both ways. Fire shot from your fingertips will cause burns at your hands, and will possibly (likely, even) cauterize your lungs if you breathe in the hot air.
Unless, well, magic.
So the answer to appropriate armor is, again... magic. Some blessing, some special, rare magical metal mined by the dwarves of ere, metal that fell from the sky, whatever.