Hello and welcome back!
Two great questions. For the first, either I'd use the Crush/Slice/Pierce values of the weapon used on the last Hunt (using whichever attribute best matches the approach taken by the player) or I'd try to find a way to resolve the situation that does not rely on dice.
For the second, I'm happy to share my thoughts. My take is that behemoths (kaiju-scale monsters) are just too big for scavengers to kill. Like in Monster Hunter World, the goal isn't to kill Zorah Magdaros, it's to redirect it to cause less harm. I see behemoths working the same way - if such a creature was heading towards your village, you'd try to incentivize it to travel in a different path, perhaps by driving some monsters it likes to eat in a particular direction, trying to hit it hard enough it decides to move away, etc. Within the game's setting, there is always a resource-dependent goal for slaying each monster - you need Artillamander bits for some cannons your village is building, you want to build a vehicle out of a Titanic Iceopod carapace, etc - and so for Behemoths, the central issue is what your scavengers want from the encounter. Make sense?