-When starting your own game, join your own syndicate. Select your branch. Go to "Employees", and select "Security Guards". On the right, you can select a Thug, Burglar, etc. Choose your Thug, hire him, then look at his stats. You can choose to beat him until his health refills. Do this a few times, then send him on his own mission. He will have an icon above his head, like an anvil or something else, and he will not take damage.
-First, set up your own syndicate. Then, go the staff screen and hire a thug. After that, press the "Punish" button (red face at the end of the health bar) until it goes so far down it replenishes itself and will not go down from full. Find a bank with a full safe, equip your thug with a gun (preferably a machine gun), and rob the bank. Make sure your player moves out of the way as he/she will not be invincible. You can do this for any member of your own syndicate, except for yourself.