Me, endescraller, and my friends Graphotism and mashumaro defeated a punching bag! The jelly one, in specific. As you may already know, punching bags have thousands upon thousands of health, which make it virtually impossible, BUT, we did it! To speed up this process, we used:
1) A tier 3 guild hall, to make our weapons do max damage
2) Our sprites both had Advanced Dark Harnesses, because jellies are weak to shadow
3) The Liquifier 3 buff, from our sprites
Mashumaro and Graphotism both used obsidian bombs (don't remember the official name) but since I don't have one, I used charged attacks from my Leviathan Blade. The entire process took maybe 20 minutes, at most. unfortunately, when the punching bag was defeated, it merely reset its health. It didn't drop anything special. It doesn't congratulate you. So, just remember not to waste your time wacking away at a punching bag, because its useless. If you want to say anything to us directly, just mail us and we may respond if we have the chance.
Proof that we defeated a punching bag:
http://imgur.com/c2npdJk
or
Yeah. That's a thing that happens.