360 partially rounded head bolt help

As a former jet engine mechanic, I can tell you that a 12 point socket is only for those fasteners that are in hard to reach spots where you need every 16th of an inch to make the turns.

You should always opt for a six point where torque is involved, even if you have to use an extension. That will save you a ton of time, money, and frustration, on problems like this.

As far as your current situation: Find the closest size that ALMOST fits. Then use (or BUY and use) an impact socket of that size and hammer it on, like someone else said. Then slow, steady, EVEN pressure until the bolt breaks free.

Do not use a regular chrome socket, those are "glass hard" and can just crack or shatter.