This is another one of those things I accidentally discovered. By picking up a sale item in one of those random shops and putting it in the pouch, the shop guards instantly appear. Defeat them. Now, place the sale item back on the floor and pick it up again (via the "footing" option) and more guards appear. Bash them and repeat as you please. The only catch is that you have to use a special skill or throw something that isn't a sale item to make the first attack. After that, you can use the regular attack.
With this, it's easy to get a ton of extra level ups, but only if you have enough DEF. Guards are typically half the level as a shop keeper so they're much easier to get. Just don't get or approach the main shop keeper unless the divine wind is about to carry you off to the next floor.
Doing this in the advanced martial arts dungeon can lead to a ton of level ups early on, but with max enemy boosting, you need a good 6 to 8 million DEF. That may seem high, but it's based on the shop keeper - the guards only need half of that. When you've got overboosted level 5 (or level 6) equipment, you really only need around level 200 to 300 for that, depending on title (use a DEF-focused one) and icons).
The only way this can be done in EX Advanced is through an irregular room, which is quite rare but these shop keepers are crazy powerful and you need far, far more DEF (somewhere around 60 million for the shopkeeper, 30 million for the guards), to the point that it's almost not worth the effort. This has to be in the form of the normal dungeon mode for it to work - you can't do it with the cosplay shop, the icon shop, etc. where the home world music plays.
At first, it'll be rather slow, but you'll level up like crazy and with a few level ups, it gets faster and easier.
