There are opposing opinons for removing nasty bolts. One idea is to heat up the bolt to expand the metal to help.break it free. Don't loosen hot, but allow to cool first. Freon or compressed CO2 Dust Off, or ice cubes can help. You only want to get the bolt to break the threads.
The other aurgument is to start cold and go hot if needed. Spray lube (WD40 or Liquid Wrench), let penetrate, firm smack with hammer, attempt to loosen with constant pressure. Torque both directions while tapping on breaker bar to get it to break free. If that doesn't work, turn the engine on to heat the block. Contract, expand, contract. Best tools are a long breaker bar and six point socket to save the heads. Kind of like a relationship. Lube it, tap it, keep constant pressure on it while you keep tapping it.
I take no responsibility if a bolt breaks. If one does, a reverse direction drill bit will hopefully break the threads free.