Nevermind re. below, I found a good hint in another posting:)
AWESOME GAME:)

I've gotten a bit stuck from time to time with running out of ammo and life later in the game, and this is my second time trying (on EASY mode too, lol), so I would like to find where all that info would be posted when I'm stuck...as I am now on level 13, out of life/ammo and no bronze key to deal with the snake,
thnx!

I was in the same situation and decided to go back to my saved previous level to stuck up on life and ammo. Again ended up without ammo but enough life to work out some strategy.

(I am a 55yr old mother of 17yr old son trying hard to understand the additive element in computer games.... and here I am getting totally obsessed with it... ha ha)

Getting back to the SNAKE MONSTER on level 13 (my life was 141 / and 1 shield) Falling down to the snake cave pressing right arrow I run as quickly as possible to get the second shield on the right. Then turning around and jumping on the first platform (this might take few attemps but eventually the snake might get caught under a cavity on the right (opposite to that last platform) This is where my victory was sweet... Jasper was jumping constantly with an unbroken stream of default ammo (by keeping my fingers glued to Z and X )

The monster didn't hit me even once before was defeated and left the brown key stacked to the wall... YAY!!!

This comment might be too late for you but hopefully will help others (I am sure there is a better solution, but I am proud of this one... I am just a beginner. Cheers.