The Crystal Slime is a lightning fast enemy which is named after its color. The best tactic is to either corner it or freeze it with Frozen Shader. A man in Raejack Village wants you to kill 10 of these, in order to get a Life Up. In Block Defence, the Crystal Slime is the fastest enemy, but has very low HP, so one or two hits with the sword is usually enough to defeat one. Use Spin Slash for the greatest effect, as the Sword shold hit most of the incoming Crystal Slimes, and your towers should defeat the rest.