"Technical mastery" refers to the level of skill and expertise one has in a specific field or discipline. It means being highly knowledgeable and proficient in the technical aspects or techniques of that subject. Full definition
They also touch upon such subjects as his belief that true artists should be able to draw so they can develop technical mastery of light and perspective. (bbc.co.uk)
Since then, he has made his name by creating psychologically charged paintings that combine technical mastery with scenes that are removed from any specific time or place. (davidzwirner.com)
Let me give you one advice: Focus on technical mastery and the weight you can lift will follow automatically! (christianbosse.com)