Well, since the counting is started by the button press, I would consider it as C-criteria (so complete failure).
The strange thing is that I've tried all of the attached solutions with the same results: the MCU resets.
I'm surely doing something wrong because it behave almost exactly even WIHTOUT any MOV, TVS or LPF installed.
Just to describe better the setup:
the DUT is a battery operated device.
one terminal of the BBQ electronic igniter goes to battery GND of the DUT.
the other terminal goes to a sharp metal point used as ESD gun and put in contact with the possibile ESD entry point (say an exposed led) while the electronic igniter is on.
Theoretically, the ESD should discharge completely to GND because the TVS clamps anything over 6.8V, so basically the MCU should not be affected by the spark.
In practical, MCU resets even discharging the spark directly on the battery GND terminal (so without touching anything in the middle). Maybe the spark rises the GND potential and the voltage on the MCU drops to zero instantly?
I'm obviously doing something wrong or missing some details.