Well, spot heating is better than none, but you do want to be gentle in the application of heat. A torch would be fine, but don't concentrate the flame at only one spot, or you'll "upset" the metal and cause a permanent warp. Play the flame around the area.
For thin sheet like that, you might also try using a chunk of copper as a backup plate. Heat this too. It should help prevent rapid cooling of the weld. That's really what we're trying to prevent. Don't use a carbon plate backup, as some carbon will mix with the weld pool and create "tool steel" for you.
What type of mig wire is it? The exact grade? Flux cored wire could possibly increase the likelihood of a hardenable alloy forming.
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