well ....
a goat's furry, and a mammal ..... 
some critters are easy to kill .... crabs and lobsters and what
not I'll toss them into the freezer first .... put them to sleep
before executing them ....
the hardest would be those big siamese snakeheads .....
their skulls are thick .... I'd bring out my camping axe if I
have to despatch a big one .... and you'd see their heads still
twitching and the eyeballs still staring at you even after you've
sliced them off ....
that's why I don't go snakehead fishing often ...
snakeheads are tough.. the traditional way is to bash the head
in but like you said, they've hard heads..
i thankfully never had to despatch one myself.. saw the
fishmonger's hugeass wound on his hand once.. he got bitten by a
snakehead 
i find newspapers are your best bet to grab hold of fish..
