dig a hole below you, place a block 2 above you.

Let a magma cube jump to the corner of your block, facing diagonally.

:- -   -   -:

:             :

:_ _   _   _:X

He'll bypass landing on the cube above you and hit you because Minecraft says so. It's like their hitbox to get you is the same size as them, but their wall and floor collision box is smaller, allowing them to get you through corners.

Sort it out. This game pisses me off with the amount of BS ways you can die from utter trash bugs.