Wait, I'm not sure what's going on in this situation.Shadow Tag Scarf enemy Shandera comes in. Since you both can switch (or so does your opponent think), he'll expect you to switch to a steel type to take the obvious shadow ball, thus, he'll just Fire Blast.
Congratulations on successfully trolling with Flash Fire. Now you are free to do whatever the hell you feel like. Sub, Nitro Charge, or just attack.
How is the enemy coming in? If they switch out for Shandera, then they obviously know you're running a non-Shadow Tag set and thus not Fire Blast you while you're still trapped to eat a Shadow Ball. I also don't know if switching a non-Flash Fire-Heatran steel type into a fire type with that high of a SpA is a common reaction.