Get a small wire brush (powered, if possible) and clean the stud threads that are exposed.
Spray nuts with penetrating oil - PB Blaster, WD-40, etc. Let sit overnight. If rust/corrosion is really bad, repeat.
You're going to need a lot of torque
and/or impact. You can buy small electric/battery impacts for pretty cheap nowadays - can be well worth the cost just in reduced frustration levels. 
Or use a long-handled wrench.
Applying heat can help but do NOT use an open flame under your car. Even a hair dryer can ignite gas fumes.
Good luck.
Edit: Forget the impact wrench. See discussion below.