Cast Time Stretch by paying ({8} magic symbol) ({U} magic symbol) ({U} magic symbol) .
Riku's first ability triggers - choose not to pay.
Holding priority, cast Narset's Reversal by paying ({U} magic symbol) ({U} magic symbol) , targeting Time Stretch.
Riku's first ability triggers, causing you to pay ({U} magic symbol) ({R} magic symbol) to copy Narset's Reversal, with the copy targeting the original Narset's Reversal.
Resolve the copy of Narset's Reversal, copying the original Narset's Reversal targeting Time Stretch, and returning the original Narset's Reversal to your hand.
Resolve the copy of Narset's Reversal created in step 5, copying Time Stretch and returning the original Time Stretch to your hand.
Resolve Time Stretch, causing you to take two extra turns after this one.
Repeat each turn.
Notes
You may pay an additional ({U} magic symbol) ({R} magic symbol) to copy the extra turn spell, and only need to do this loop during your fourth extra turn.