You can do solo training on a live punching bag safely. Build a skeleton repair bed and put it in a room with a number of regular beds. Have decent toughness before you start training, so you can survive getting beaten down. Have passive switched on all the time, the AI will beat you silly as soon as you regain consciousness normally but with passive on they won't aggro immediately. Turn stealth on after you're knocked out so you can move out of the AI's line of sight when you get up. Hop into the skeleton repair bed to quickly patch up all of your injuries simultaneously (works on organics) without wasting first aid kids and then get into a regular bed to heal the damage and your training can keep going on and on until you finally bring the other guy down with minimal downtime between fights.