Boomer's Napalm is a rolled skill die, which then acts as a counter.

Say you roll it and 2x3 comes up(lucky!).  You would place the die on the targeted Baddie, and at the start of the Baddie's turn, the Baddie would take 2 damage.  The die would then downtick to 2x2.  At the start of the Baddie's turn next round, it would take 2 damage, and the die would then downtick to 2x1.  At the start of its next turn, the Baddie would take another 2 damage, and then the die would be exhausted.
Backlash will only hit Boomer if this is the size that comes up when the die is rolled.  The counter will not tick down to the backlash side.