Not sure if this is intended or an unexpected combination of mechanics, but currently it is possible to farm free XP with the Starfish trinket from Lost Trinkets. It adds XP when you hit (not kill, just hit) an enemy. And MmmMmmMmmMmm adds a Target Dummy, that you can punch to test DPS. The result... well, it's free XP estate ^_^)

Here's a video showcase

Reported here; owmii/Lost-Trinkets#87
It's likely this won't be fixed tbh, as it would be nearly as easy to make a mob farm and hit the mobs.