Master

an artist of consummate skill

a person who has general authority over others

a combatant who is able to defeat rivals

directs the work of others

presiding officer of a school

an original creation (i.e., an audio recording) from which copies can be made

an officer who is licensed to command a merchant ship

someone who holds a master's degree from academic institution

an authority qualified to teach apprentices

key that secures entrance everywhere

be or become completely proficient or skilled in

get on top of

have dominance or the power to defeat over

have a firm understanding or knowledge of

most important element