Use a good quality hex/allen bit on it. Smack it hard with a hammer a few times (the reason to use a quality allen bit) and then use your breaker bar or impact on it.

If you can't get it out and need something in the meantime, run a droplight under the hood with a 100watt bulb and leave it on overnight (hood closed). It will keep it warm enough to make starts a little easier, but may run that electric bill right on up.