Every time you finish a challenge for the first time you will receive a star. Every level has 4 challenges. The first of which is always "defeat all the monsters". After winning that star, when selecting the level you can hit
to scroll right and select other star challenges. These vary by level but they almost always have a time trial. Some examples of challenges you have to complete are enemies have double health, deal extra damage, only words that start with vowels deal damage, only words that contain 2 corner letters deal damage, etc.
The 4th star in each level is a super hard challenge mission. You will be able to defeat the first 3 stars in each of the first 10 levels before being able to comfortably complete these hard challenge stars.
Complete 29 of these basic stars and the achievement is yours.