MEGSTIEL would be great for this (for perhaps too obvious a reason) but you can mix up the cast however you want. DEAN AND SAM, IF PRESENT, SHOULD TAKE A BACK SEAT and let other characters do their thing

Castiel as The Unicorn/Amalthea, Meg as Prince Lír, Lucifer as King Haggard, Crowley as Schmendrick, Charlie as Molly Grue.