Dynamite has done 15 damage versus:
- Winged Hunter (Arkaim, September 2013)
- Mutated Bum (Al, June 2012)
- Ruins Octopus (PenguinPyro, April 2012)
These all post-date Engineer Explosives (December 2013), and Arkaim had that skill when he fought the winged hunter, so that's an easily tested possibility. With no message change and dynamite not being crafted it seems unlikely, however.
More likely, the dynamite does 15 damage against any opponent with negative fire defense.
The Mutated Bum has -3 fire defense
The Ruins Octopus does not have any relevant logs to test fire defense.
The Winged Hunter might have negative fire defense — I came up with something like 0-2 defense before I encountered contradictory results and realized he had reactive defense I wasn't accounting for. Someone else can tackle that if they would like.
Test with enemies with confirmed negative fire defense:
In the beginning, men were mortal.