I had a deck with Focused Burst, NitroX, Frag Grenade, and Rain of Bullets and never got the technique. As soon as I switched from Focused Burst to Single Shot I started getting the technique after battle. Shotgun Blast works too. I suspect that you specifically need to use a single shot technique in conjunction with fire techniques to get this, and that burst techniques don't work.
Smoldering Shot / Discussion
This is the discussion related to the wiki page Smoldering Shot.