Auto means the system will automatically assign characters for the battle. You can unlock pro mode by completing at least 10 battles. In Pro mode, you decide which characters you want to use in battle.
When setting up your own choice of characters, follow the rock, paper, scissor principle.
(Shotgun beats pistol, pistol beats rifle. Rifle beats shotgun, shotgun beats pistol. Pistol beats rifle, rifle beats shotgun.) Don’t forget to take a good luck at the choice of weapons your opponent has, the amount of damage and health as well as the character level.