Your first task will be finding a Mythical Monster. The type of monster you get in a level depends on chance. The percentage odds of getting each type of monster is shown when you click on the level your want to play. In all of the worlds, Mythical monsters do not even have a chance of spawning until the last 5 levels of each world (11-15). Level 15 of each world has the best odds (10% chance) of spawning a Mythical monster, so try that one if you are having trouble. Evolving a Mythical monster works the same way as evolving any other type of monster (see the "Pure Sugar" achievement), except it is more difficult as you need to feed it more candy. It is best to use a good monster with helpful and plentiful boosts when attempting this.