Note: Looting fallen combatants is strictly against the Battle Rules of the Arena. Therefore, capturing Shimmerstrike is not possible without using the console or the aid of a mod or hack.
You can obtain this weapon by using the Skull of Corruption on your opponent and killing the clone.
Note: If using the Skull of Corruption glitch to gain the sword, a glitch may occur when loading a saved character who has the sword on his/her person. Solution is to place the sword in a container or drop on the ground before saving.