Playing.
Ok, to play you need to chuck everything you've learnt so far into a bundle and sift in some rules. Ok, here is a table showing what something can kill, can't kill, and block.

Thingo It kills... It blocks/sends back... It, sadly, can't kill...
Gun A reload A gun A sheild, A rebound
A reload Lack of bullets Nothing A gun, A sheild, A rebound
A sheild Nothing A sheild, A gun, A rebound A reload
A rebound A sheild, A gun, A reload A sheild, A gun, A reload Nothing

There, ahh, that took some effort.