Heating it is only going to make it swell up and get tighter, but that expansion and contraction may break whatever crud is holding it.
If it were me I'd drill the head off then drill through the bolt with the correct tap size drill and if the remnants didn't come out after that I'd run a tap through it.
They also make left handed drill bits that work well for this sort of thing. Seems that drilling out the majority of the diameter weakens the grip and the remnants will spin out on the bit. If it's not a blind hole you can use a regular drill bit.
I'm not crazy about easy-outs, they actually cause the bolt to swell up as they dig in and god help you if you break one off.
That's my two cents.
