In an ideal world every film that established a popular character/s or theme in a series would be better than or at least as good as the film that preceded it and there are rare exceptions where this is the case, the "Harry Potter" films being a case in point.
Harry Potter benefits from being based on a series of novels that had already achieved phenomenal popularity before the films were made, largely down to J.K Rowling's carefully crafted world that had a story arc spanning several books.
With the "Gamera" series, in much the same way as it's counterpart, "Godzilla", the products of rival studios, it has been a case of an established successful template / formula and developing variations on that, and this puts a great deal of the responsibility on the central idea for any film in the series - reflected in the script - that determines how the film will play out.
30-odd years on from the original 'Gamera" film that introduced Japanese and later, world, audiences, to the giant jet-propelled amphibian "Kaiju", "Gamera 2 - The Attack of Legion" already has a legacy that charts an uneven graph which by the late 70's and early 80's looked as if it signaled the end of the series as attempts to re-boot the character by making it more child-friendly resulted in anodyne plots that became an excuse for the main draw - watching giant monsters slugging it out and causing mega-destruction.
That said, I was prepared to be disappointed, but still, curious - there's no doubt that a key driver in re-booting the "Gamera" (and "Godzilla") films was the announcement that a new "Godzilla" film was to be produced in Hollywood and indeed this film roughly coincides with the 1998 Roland Emmerich film - suddenly the visual effects and story stakes had been moved up a notch and there is the sense that Japanese audiences would compare home-grown cinema unfavorably with anything coming from Hollywood.
All this works in favour of the film, which pits Gamera against an alien insect life-form that arrives on Earth via a meteorite - a pretty standard formula it has to be said, but for a change the build-up and pacing of the story is such that it avoids cutting to the chase, ie fighting monsters, too early on, saving the best for a truly titanic face-off at the film's climax.
The visual effects in the film are first-rate, being a combination of CGI and "Suit-Mation", sitting within convincing miniature work, and is, I think, some of the best I've seen of this often derided medium, the result of an often seamless blend of different techniques at a point in time before visual effects became wholly computer generated and this is one of its key assets.
There are some inevitable minus points that perhaps would be lost on a non-Japanese audience, in particular the point in the climax where Gamera summons up some kind of mysterious "Earth Energy" as he faces certain defeat and effectively saves the day by destroying the almost unstoppable "Legion", but what makes Gamera interesting is that he is not infallible - he has a few tricks up his sleeve, such as his fiery breath and being jet-propelled but other than that he relies purely on brute strength and is often massively out-matched, as in this film, and anyway by the climax all attempts by the military have failed to stop Legion, so perhaps some kind of narrative device had to be employed for Gamera to emerge the victor, and more importantly, to leave the audience with the oft underlined satisfying message, which is that if we look after nature, then nature will look after us, and is what Gamera is all about, after all.