If I don't use a shadow ele for training, I'll go tame two swamp dragons and have them fight each other. Then I'll get my one slot pet to attack one of them and keep vetting. Swampies have decent hp so it works well. Can also just fight another of the same type of pet as you want trained and let them both raise up in skill.
Shadow eles usually spawn with too high of skills to be much use for fresh one slots, and you need something else to tank. If you can disco and can dig up a low skill ele, then disco the ele and let the one slot pet fight it after the tank goes in. If I know anyone who needs a cu sidhe trained in healing, I'll offer this option up when I need a one slot worked. Disco the cu sidhe and the ele then send in my pet. Cu will get healing and take care of the damage and my pet gets skills.