With the ally and a room with 2 entrances spaced far enough apart and the shop owner also far enough away, I can easily fell about 1000 to 1300 shop guards on a single map. Without the companion, however, I have to repeatedly place a piece of equipment, one for sale, on the ground and pick it back up again to respawn the shop guards, which requires the use of limited SP. However, in this case, I kind of figured out how to do it without the companion, but it requires the hallway entrances to be so close together, 2 panels apart, that it's not likely to occur often. However, one shop guard every 2 turns is better than 2 shop guards every 6 turns so this route is certainly faster.

To do this, you need to spare an enemy that has a range of less than 3 panels, so it can't attack. Lure it into the shop. Now, pick up a sale item, putting it in the pouch. This will spawn the guards. Defeat one (or both) of the guards but not the other enemy you lured in. Move to where one of the guards used to be and place the sale item on the ground. Pick it back up again to spawn the guards again, except, since you're taking up the space where the guard is supposed to spawn, that one doesn't spawn. Now, attack the guard without defeating it. Consider lowing accuracy (e.g. equip a weaker helm) or tossing a piece of nonsale equipment or one of the many mega beefy drops at it. This will cause the guard to move. Since you're hogging up the spot where the guard is to be, a new one will appear. At this point, toss the lured in enemy into the hallway right next to the guard that just spawned. From here, raise the accuracy again and begin attacking. If done right, every 2 turns will spawn a new guard that you can defeat for near-endless leveling up. The divine wind kicks in 1500 turns in and you've probably used about 300 to 500 just to set things up.