A restro specced druid will probably not be able to kill you in the game
Star-fire can stun you and moon-fire has incredible burst damage. If you see they have blue light around their hands use your counter-spell to lock them down. If your counter-spell missed try to survive Dunegeon Fighter Gold by absorbing their damage.
If you are fighting alone and do not have any healing, dampen magic is very useful as well as Ice Barrier and Mana Shield if you have enough mana. If they stay and range and cast use your heavy nukes like fireball to damage them.
If they run up and try to hit you they are probably trying to regain mana and need to be avoided, use Blink or frost nova to get away. If they run out of mana or you time your Counter-spell right, you can take them down fairly quickly.
A restro specced druid will probably not be able to kill you, but it is very hard for you to kill them. If you can not shut down their healing with Counter-spell the odds are you will run out of mana before they do.
If you see they are casting a non instant heal like Healing Touch or Regrowth silence them with Counter-spell. You can sometimes trick them into doing this by using Spell Steal to steal their heal over time; blast them with an instant cast, and panic them into using a powerful heal.
Once you have shut down their healing blast them with everything you have. Arcane Missiles is my personal choice because of its high DPS.