Triflow works okay. When I worked in a machine shop, we used an acetylene torch and a rosebud tip to heat old bolts in engine blocks to a light cherry red. Then you dab on some paraffin wax. The wax will liquefy and quench the heat while filling the void from dissimilar heating. You will harden the fastener with the quenching and you need a place to target the fastener directly. However, this technique works so well that it is common to go from a rusty blob that looks like a parasitic ferric lifeform has merged with the host to something that can be unscrewed by hand after it has cooled. You just risk damaging finishes with this one.

Using a combination of triflow, hot/cold cycling, and janky leverage, are the ways it is done in a bike shop. GL