Say we have a pterodactyl-wyvern or a huge dragon-anaconda, maybe something like a t-rex, or a sand-crocodile, or a slime--- you get the idea. Assuming one wanted to make a fictional manual with tactics on how to effectively and safely hunt these beasts, as well as fight them in solitary if one is forced to... how would I be able to know what kind and size of bow/crossbow, or what caliber of the gun, or how much force of a spear (and if a human, as well as the spear itself, can withstand it) and if that would be enough to kill the beast? I'm talking about every possibility, be it destroying an organ, smashing the skull, creating enough of a blood loss in the monster or tearing muscles/ligaments and breaking bones to the point it would die anyway. What should I have in mind? Is there a "formula" for it that I can apply?
Hunters and hunting theorists have developed a number of formulae for calculating killing power: https://www.chuckhawks.com/killing_power_formula-2.html
You can take any of these and plug in your game. However, if you get weird results, know something went wrong. These are for guns.
An important factor is that there are different degrees of "kill". A nicked artery can kill something, but it might take a long time. Modern hunters aim for a quick kill that doesn't let the game run far and keeps the hunter safe from retaliation. Prehistoric hunters had to do with lower standards.
With cold weapons, i.e. bows, crossbows, spears - you can use short bows for small game, and the best available for everything else - it still won't be overkill. Human muscle energy is well below that delivered by firearms.
It's all about placement.
Elephants are about as big as trex's, and people have hunted them with spears. Hit them right and they die.
If your aim is shitty, you need an elephant gun, which is basically a big caliber bullet. Smaller bullets did kill elephants, but you might need 30-40.
Trial by Combat
There's a very simple way of determining the best way of how much power and penetration it takes to damage a creature.
Step 1: Capture a dead specimen.
Step 2: Experiment on dead specimen, noting what forces and what angles it takes to destroy certain parts and certain organs
Step 3: Attempt these techniques on living creatures, note what works best, and refine approach.
Yes, this is slow and methodical work, and doesn't allow you to just plug in information into a formula and emerge with the correct answer, but on the other hand, real life is messy, and there's no substitute for raw data and raw experiments.
humans used to kill mastodons with flint tipped spears and a fair degree of cunning.
destroying and killing are very different goals.
To be able to calculate the power needed for Destruction one needs to first specify the form of the power: mechanical, electrical, burning etc
for a simple example: there are formulas for calculating the amount of fuel needed to completely burn a body of X mass and Y percentage of water - consult any crematorium near you
now killing on the other hand requires a knowledge of the ways that one can end an organisms life. damage the brain, the heart and now matter how large or strong the beast will die. With very little power needed.
The question needs to be reformulated