It's possible to become invincible if you have a friend who is willing to lend a hand. Start by creating a full set of Diamond Armor, which you should place in your inventory. Now place a Dispenser and put your Diamond Armor within the Dispenser. Next, have your friend smash the Dispenser. Grab your armor and place it back in your inventory. From there, equip the armor. Now have your friend hit you and the armor will disappear. However, you should now be invincible.