I do believe in using enough gun.
The .22LR has been used to shoot nearly every beast on the planet. Some way unethical but still been done. The .22LR have the potential to kill an elephant, its been done, but to me only by an irresponsible idiot.
Then there is the right gun but too soft a bullet. The example that I know true is a shepherd who used a Webley revolver three shots on the head of a distressed sheep. The following day the sheep had recovered with three welts on its head, absolutely fine. The lead bullets far too soft and without much velocity for some reason.
Interestingly in the not so distant past British Officers, Police and Military, carried Webley pistols not just for the bad guys, but more commonly used to shoot rabid dogs and put maimed horses down.
Red Indians used quite low powered bows to bring down Buffalo. Just enough power to penetrate to the heart.

Shot placement and enough to reach a vital organ is all that is required. More gun is often just to compensate for error in placement. However, some tough animal's will for life is high and it takes a lot to bring them down; the African buffalo notorious for how tough they are which makes them so dangerous.

Airgun power varies considerably from only a few ft/lbs to military weapons of great power. I've had a grey squirrel jump at my face having five .22 pellets in its body fired from a Webley MKIII which were know to be pretty powerful. Its why I only head soot them now. Export rifles can easily be a lot higher than 12ft/lbs, your Diana example may have twice that power??? Enough to deal with a dog sized animal. More importantly is it legitimate to the law of the land??? Heck, Halal hunters do not shoot to kill as only a slitting of the throat of say a deer is religiously acceptable for human consumption.
There are some really weird ways used to harvest nature's bounty around the world, many are not legal in the UK. Many would make me uneasy.

So I'm not sure where we are going with this thread now. I know what I find acceptable, and keep to that. What other cultures do is up to them.