Im not an Urza expert, but I have dabbled
Overlord of the Boilerbilges can either be a 6-mana 5/5 red creature that can remove something (or hit face) with its attack trigger, or just a burn spell for 4 mana
Ashroot Animist Exuberant Firestoker Faldorn, Dread Wolf Herald Fleetfoot Dancer Jinnie Fay, Jetmir's Second Kirri, Talented Sprout Kitt Kanto, Mayhem Diva Knotvine Mystic Leafkin Avenger Nikya of the Old Ways Rakish Revelers Rocco, Cabaretti Caterer Rocco, Street Chef Roxanne, Starfall Savant Sarkhan, Dragon Ascendant Sprouting Goblin Szarel, Genesis Shepherd Tana, the Bloodsower Wild Cantor Yarus, Roar of the Old Gods Zhur-Taa Druid
Were talking cards like Tasigur, the Golden Fang, Soulfire Grand Master and Alesha, Who Smiles at Death
If the creature Bitter Chill would enchant is an illegal target by the time the Aura spell resolves, the entire spell doesn't resolve
Adds: Nullpriest of Oblivion