Find a NPC that will not respond. Knock them out and drag their body behind a barn or some area no one will see you. Use your fists to keep hitting them for easy and fast XP. It is recommended to have enough money to pay off the bounty if you are caught. Otherwise, just put all your loot inside inventory somewhere, then go to jail to restore your reputation. One place you can do this is in the village of Sasau. Find the gravedigger and knock him out, then drag him into the water where his body will float and no one can see you. Stand so that he is level with your character, and you can punch him. You will keep getting Unarmed, Warfare, Vitality, Agility, and Strength XP while repeatedly punching him. Note: This can destroy your reputation. It is recommended to get the Infamous perk to raise your reputation faster. People will stop talking to you when you have a low reputation. Additionally, you can use weapons with this exploit, but it is a little trickier. Fists do not normally kill, but any of your weapons will. You have to find essential plot-important characters that cannot be killed. There are a few in every town and city, but they tend to live in areas that are hard to access secretly. Essential characters cannot be killed, so they will be knocked out by your attacks and will eventually get back up. It is much harder to stab essential NPCs in secret, so you are more likely to (eventually) get caught. If you worried about your reputation, use the Infamous perk to make it easier to increase your reputation, and only try this exploit in cities that do not really matter to you.