The whomp is made using three layers. Every whomp needs it's own three layers and events dedicated to it, and unfortunately I cannot see any way around that.
The killing is done via a 2x2 resizable block, two normal blocks, and two lava blocks. All invisible and put down with auto allign. The way I have postioned them is a bit buggy, so I would suggest placing the lava a little lower.
I am aware some frames are utter garbage, but I made this 2-3 years ago, and it is more of a proof of concept. Fell free to improve them if you like. :)

To see the killing mechanism, remove all files with block in their name from the folder.