firstly its a b movie, and a very good one at that when you watch it for what it is, plus as said Denise Richards.....
But, your actual questions, why should bugs, massive, massive massive bugs not take loads of bullets to die, they are huge, bullets are small, not sure what makes that completely unbelievable. LIkewise on tactics, on some of the planets its just baren landscape, nothing to hide behind, not sure what else they are supposed to do exactly?
Plus, when you have mini nukes you probably have the right to run in a bit trigger happy.
But the film has a sliding vagina for a bug leader, why take it seriously.
One last thing worth mentioning, compared to the follow up films, its triple aaa blockbuster quality movie. 2nd has some nice boobs iirc, the 3rd is a complete and utter joke. Jolene Blalock(chick from the Enterprise series) is so embarrasingly bad in it, it makes casper van dien in the first seem like the De Niro of his generation.