I agree, it seems to be "unverdrossen".
Also, first line, isn´t there a "er" behind"Regiment", which would make it plural "der Regimenter"? Otherwise "des Regiments" is the gramatically correct singular in this context.
Lastly, "...mehrfach den Maschinengewehr kaltblütig und entschlossen standhielt..." doesn´t make sense. But you are probably right, as i only see a "den" or "dem" in front of "Maschinengewehr" and no "mit". It may be a forgotten word by the author or a misconstucted sentence, but in my opinion he meant "...mehrfach mit dem Maschinengewehr kaltblütig..."
all the best,
Gerd