There are 40 total levels in the game's story mode. In each one you can earn 3 yellow stars and 1 crystal star. You must complete all 3 of the levels normal yellow star challenges to unlock the crystal star challenge. You can scroll through which challenge to play by hitting
when selecting the level.
These crystal star challenges are much tougher than the normal challenges as enemies are stronger, have more health, and they always have special conditions to the battle such as "takes no damage from words that contain the letter E", or "Takes triple damage from words that contain 2 bottom row letters". Sometimes these conditions help you, sometimes they hurt you. If you want to cheat, use
http://wordunscrambler.com/ to find all possible words and just spell the longest one.
Once you have done 20 of these stars, you are officially 50% done with the story mode.