Defeat Urul.

You will encounter this Boss on Wave 100.

To save you the grief on the wave difficulties, feel free to play on STORY difficulty as The Crucible DLC does not have difficulty achievements that stack although this will affect your score overall when it comes to purchasing brand new items from Targon. Save your Havoc special ('LB' + 'RT') for the Boss waves which they'll appear on Wave 25, Wave 50, Wave 75, Wave 100 and Wave 101 but some waves spawn tough mini-bosses as they prove to be a challenge. So use it sparingly, and use a Havoc shard if needed. Depending on which difficulty you have it on and which enhancements you have equipped, the Havoc form special should make quick work of the Bosses of their respected waves. Also, be ready for a fight if you're using the Havoc form against Urul; he will trigger an animation-scene when his HP is down to 30%, taking away your Havoc form.

Please see "Quite Proper" for a recommended list on which enhancements to use for the 101-wave run.