360 partially rounded head bolt help

12 point sockets are made for 12 point bolts.
Use a 6 point always when you can. Get a fine headed ratchet if you need the sensitivity.
Clean the debris out from around the bolt head. (which probably caused the problem to start with).
Get a GOOD 6 point 18mm impact socket with extension and DRIVE it on with a BFH until it bottoms out. Don't be afraid to hit it hard.
Use a breaker bar on the extension/socket to loosen the bolt. If you don't absolutely need the extension, use the breaker bar on the socket alone. Some finesse is needed here, make sure you are square to the bolt and use even pressure, and use a steady pull as opposed to a jerking motion. Make sure your arms are at a 90 degree angle to the breaker bar.
It should pull right off.
If you twist the head of the bolt off, take the head off and remove the stub with a pipe wrench and heat where the threads go into the block.
You realize that the head will probably need to be resurfaced, right?
Check it after you have it out.