Whenever you come across a battle with surrendering cumans or bandits, wait until their opponents walk away or chase after those who flee. Then you go to work and start to pickpocket every crouching cuman/bandit. Steal one item at a time for maximum experience.
You can easily succeed with three to five attempts per person. Keep your bow ready if they notice you, although they are not likely to do so the first three times.
Keep in mind that when you pickpocket or kill surrenders, they count as civilians, despite being cumans and bandits – if you want your crime record clean. If you don’t care about that, you can finish them off with a stealth kill to gain some stealth experience.