I like the meme, but try making the second thud softer and if you use synfig for animating its free and lets you make better animations. Like making the monster a puppet so you can add some expressive body language.

Edit: Audacity can be used to that effect. And filmora9 CAN reduce audio although you could manage with audacity since its free. My suggestion would be to record one slap and make a duplicate that's just a touch softer.