Starfield:Boxing

Boxing is a novice tier physical skill that increases the damage you do to with unarmed attacks, decrease oxygen from power attacks, and finally adding a chance to knock down opponents at its highest tier.

Skill Increases
Boxing challenges requires you to kill enemies with unarmed attacks. One method to expediting this process could be to weaken them first with different weapons, before switching to fists for the final blow. However, it would be wise to dismiss your follower before doing this, to prevent them from finishing off enemies you were going to with your fists.

Another angle would be focusing on weak enemies, such as some of the various random Creatures that inhabit planets. Several creatures have extremely low health by default, which should make them relatively easy to kill.