The best scenario is to use it when you have the advantage and can also sing it with one of your characters
As long as you have a Mountain (which you should), all creatures you control have Haste, letting you quickly put out damage from creatures played from your hand
Invasion of Segovia Invasion of Segovia is another battle that really needs to transform for you to get the most out of it
With truly mesmerizing artwork and set design, Urzas block is a style standout, and is likely one of the best sets of all time simply from a gameplay standpoint (broken artifact-based combo decks notwithstanding)
Nonetheless, the fact that it's at its best when inside of Agatha's Soul CauldronAgatha's Soul Cauldron tells you a lot about the limits of this mechanic
Of course, these are loose guidelines (if green-red showing up twice didnt make that obvious)