How does Stellarys's Shadow Magic clone work?

Think of the clone as a flat bonus on your own output rather than a second unit you control. It does not need aiming or a separate rotation. It follows what you do, so the way to get value out of it is simply to keep attacking. Because the copy is a portion of Stellarys's damage, the clone gets stronger as Stellarys does. It scales with you instead of having a damage number of its own.

The mimicry is not limited to basic attacks. Abilities get copied too. The 20% figure is the one attached to basic attacks, and that is the part you can count on when you work out what the skill is contributing.

Stellarys fights at range and has mobility folded into its attacks, which suits a skill that pays out per attack. Repositioning is not dead time here. You can move and still be feeding the clone something to copy, which is what keeps the 20% ticking over instead of sitting idle while you reset your spacing.

Not settled: The 20% rate is stated for basic attacks only. How much of an ability's damage the clone reproduces is not settled.