Instructions
Max out your Support stat as you play the tech mage. You need to have it at level 20 to achieve Deadly Aim.
Get your character to level 10. You get to advance your skills and statistics with experience, but Deadly Aim is a prohibited skill until you reach the required character level.
Increase the Deadly Aim skill. Deadly Aim is a passive skill, meaning that as long as you have the skill, Deadly Aim is active and working.
Equip a sidearm, so that Deadly Aim improves your character's damage abilities.