HI! I was wondering what modern light infantry firearms would you recommend for killing giant monsters around size and weight of elephants but with agility more akin to cats. I was thinking heavy round assault rifles and or grenade launchers.
Well, not, “light,” but I kinda suspect you mean, “small arms.” The first thing that comes to mind is a .50 anti-material rifle. That’s not just because I did an ask on the Barrett AM rifle a few days ago.
With something that nimble, you wouldn’t want to get within half a mile of it, if you didn’t need to. And, because of how sound works, at those ranges, it wouldn’t even hear the gunshot before the round connected. (Technically, it would never hear the actual gunshot, just the bullet breaking the speed barrier.) Depending on how the critters are put together, a high-explosive round might be the best payload, but I don’t know how well their accuracy holds up at long ranges.
Getting close enough to use a grenade launcher (usually around 100-200m) doesn’t sound like a good idea. At least not if they’re that fast and agile.
(For reference the M203 under-barrel grenade launcher is accurate up to around 150m, beyond that you can still put a round general vicinity of over there at up to 350m.)
By, “heavy round,” I assume you mean automatic rifles chambered in 7.62mm (and some other .30 rounds), at which point, that’s usually a battle rifle. I mean, it’s possible you might get the desired result from riddling the things with an H&K G3, but getting that close when you don’t need to be still sounds like a bad idea to me.