Wait For It
Kill 10 enemies with flint-triggered detonations
This can be done right at the start of the game. First you have to pick up the dynamite on the right hill after you destroyed the vehicle. Walk past the first two enemies and take out the guy that’s coughing.
Now make a savegame (progress is not lost after loading) and place the dynamite next to the guy that’s standing in front of the cannon. Place the flint right on top of it and hide.
The explosion also kills the two guys standing next to the cannon. Load your savegame and repeat 3 times.
Tip: You actually don’t even have to place the dynamite as close to the guy. when i tried this first, i wanted to put it as close as possible and walked too far and he saw me. Then i found it works just fine if you take a step back ;).