I am a little confused on your story. Why where they lifting your car and how? If you meant they where trashing it or hitting it then your simple answer is the following.
1. Go to Wal-Mart or Dicks Sporting Goods and buy a real nice baseball bat.
2. Find the person that did it, beat their ***, their dad and moms *** and any sisters or brothers *** that they may have. Then **** the person that dented its girlfriend.
3.Set their house on fire. While trying to put it out with gasoline.
4. Get a dent popper and fix the dent.
Thats how i would handle it