CHECK OUT TECH SHOP. ever had a rounded bolt that no mater what you take to the head of the bolt to remove it just rounds it or mangles the head. then not really know how to get it out and not want to mangle up what it is threaded into. well I had one recently was rounded out and it was a oil drain plug that needed to come out without damaging the oil pan or the threads.
Step 1: Square Up Bolt Head
use a grinder to flatten up each side in order to get a crescent wrench on the bolt. some tips are try to make both sides of the edges as parallel to each other as possible it will make it easier to get the crescent wrench on it. try to leave as much meat on the bolt head as possible but the flat edges will have to be a certain length in order to get enough leverage rather than rounding the corners again. because as you can see the first time I grinded it under the car it wasn't parallel and I lost leverage so I just ground it down until it worked then it came out.
Step 2: Use Crescent Wrench or Vise Grips to Remove Bolt
the tool needed to remove the bolt needs to be adjustable. adjust the grind surface of the bolt until you get grip. now after you take the bolt out you can straighten out the grind again if you wish to reuse the same bolt otherwise replace the bolt.