Here are my tips:
1. A good quality tool. Cheap hex tools are rubbish when doing heavy work. Spend $25 and buy a good quality single hex key socket to suit the size bolt you have, if you don't have one already. The last thing you want is a rounded off tool/bolt or broken knuckles.
2. Lubricate the bolt. The best penetrating oil is made yourself by mixing about 80% auto trans oil and 20% acetone. It's super thin and the acetone carries the oil into every last gap. Try to build a dam around the bolt with something so that it pools and has time to soak past the threads. See if you can access the back of the bolt to get directly to the threads.
3. An impact gun is designed specifically for seized/frozen fasteners - that's why mechanics use them so much. The rattling action shocks/shears the bolt thread apart without fracturing the metal body of the bolt or tool head (it will break off if you rattle for too long, but short bursts are ok).
4. Heat. If the bolt is corroded or has too much loctite on it, you'll need some heat to thermally shock the thread, or soften the thread compound. It doesn't need to be much heat (needs to be just too hot to touch), but make sure it has time to conduct through the component to the thread interface. Put your oxy-acetylene kit away... a hot-air gun is enough, and won't start a fire/melt rubber.
5. Cold. If heat alone won't do it, hit the bolt head with freeze spray when the component is still hot. The thermal shock from the temperature difference will be greater.
6. If you still can't get it off, you have three options.
a. Use a long lever and break the bolt, or cut/drill the bolt out and repair the thread. There are plenty of thread repair guys that will come to your home to do this for you. If you think the job is awkward, I guarantee you that they've done a harder job before. I swear these guys have arms with four elbows.
b. Remove the whole component (if possible) and get better leverage on the bench. Depending on the part this may not be feasible.
Good luck. Let me know how you go.