There are no exploits like this to get more experience, that I know of, but there are three places where you can trigger opponents to spawn, or avoid completing a task to let them keep spawning. The first is in Maxos temple, probably when you are about level 15 and completed much of Broken Valley.

In the mirror room test, where you have to find the difference between two sides of a room, straight in from the teleporter and to the right just past a short wall, there is a barrel. Each time you open the barrel, three opponents will spawn; if you open/close the barrel a few times quickly, they will all remain in about the same place, and you can use an area effect spell, or Explosive Arrows / Splitting Arrows to take them out faster. They don't give much experience individually, but taking out groups you can level there, slowly.

The Developer's Cut version of the game has a Developer Mode cheat window you can use to give yourself experience (and stats, skills, equipment, etc). If you have the PC version of D2:ED, you may have or be able to upgrade to D2:DC (see here for details, in English).