I discussed another type of "weaklings with weapons" in 2013's OUR ARMIES AT WAR, WITH MONSTERS. Though a lot of "giant monster" films are combative primarily in pitting two or more behemoths against one another, there are also those in which the primary conflict is between one behemoth and the amassed armed forces of a particular country.
KING KONG, the first major "giant monster" film, concentrates the early part of its narrative on showing how Kong is "king" over all the rival creatures in his own domain, but then concludes by having him shot down by American biplanes. 1953's BEAST FROM 20,000 FATHOMS, whose filming might have been encouraged by the successful 1952 re-release of KONG, came up with a giant reptile. But the filmmakers knew that the amassed power of the U.S. military could've blown away any old dinosaur, so they had to come up with a reason for the military to avoid attacking the critter directly. 1954's GOJIRA reversed that conceit. Whereas an atomic bomb simply woke up the Fathom-Beast, it both awakened and empowered Godzilla, making it possible for the giant monster to stride fearlessly through cannonfire, airplane missiles, and electrical fences. In my review I mentioned that Godzilla was, on one level, the symbol of any martial enemy of Japan, so that in a strange way, this most Japanese of monsters bears some resemblance to the forces of the Allied invaders, whose might is represented by the atom bomb itself.
Later Godzilla films always followed Kong's trope of pitting the Big G against other colossi, but the 1950s and 1960s included a smattering of giant-monster films in which the monster's only opponent was humanity's armed forces-- American in THE GIANT CLAW and IT CAME FROM BENEATH THE SEA, British in GORGO and THE GIANT BEHEMOTH, Danish in REPTILICUS, and Japanese in such non-Godzilla films as RODAN and MOTHRA, and THE X FROM OUTER SPACE and GAPPA THE TRIPHIBIAN MONSTER. In addition, DC Comics made a major contribution to the "soldiers vs. dinosaurs" trope in its "War That Time Forgot" series, which ran for eight years in STAR-SPANGLED WAR STORIES.
I've read none of these stories, so it may be that none of them muster the necessary "spectacular violence" necessary for the combative mode, the same way BEAST WITH 20,000 FATHOMS fails the test. the 1950s flicks TARANTULA and THE DEADLY MANTIS also fall short for one reason or the other.
Now, on occasions, human soldiers are aided by some weapon that's just as metaphenomenal as the monster. The original Godzilla is vanquished by the "oxygen destroyer," and the Giant Claw, one of the few American monsters immune to military weapons-fire, is undone when scientists reverse the protective "meson-field" about the creature. But the use of an "achilles heel" weapon may not give rise to the combative mode, either.
In the end, the reason that the nameless soldier-hordes can qualify as combative entities is because there is a necessary connection between the warriors and their weapons, whose use the soldiers have implicitly mastered. But there must be diegetic evidence of such mastery, which I find in REPTILICUS but not in DEADLY MANTIS, as discussed here.