Hire an NPC and simply kill him or her. This works well if you hire Otis, Elmo, Lareth, or any high level character. Once you have earned an Elemental Gem (easiest with Earth, Air, and Water), use it to summon a demon. Use a Rogue or Fighter to melee the demon with full attacks and kill it before the duration expires. When it dies, loot it to retrieve a free Elemental Gem for another character, at no risk, plus gain the experience for killing the creature.