The text on Kaza, Roil Chaser reads: The next instant or sorcery spell you cast this turn costs X less to cast, where X is the number of Wizards you control as this ability resolves. In the same vein as Vadrik, you get the total cost of the spell plus kicker, and then reduce the total cost
Last but not least-ly, it also ramps you converting three red mana into four red mana
If you really wanted to reinforce Unions as a whole to compete with the current meta: I would say something like "Target 1 Union monster from your hand, Deck, or Graveyard: Special Summon 1 monster whose name is included in the card text of that card from your Deck or Graveyard
This option allows you to replay Story mode with all previously collected cards and the same World Championship progress
While they are prevented from attacking until there are seven or more cards in exile, he does encourage these effects with an ability that lets you draw cards when one or more cards go into exile from the graveyard or the battlefield
Then all of the chosen permanents are sacrificed simultaneously