Wikia

Simple Method

Chose any level. The first six are logically the "easiest."
Clear the first wave however you would like, if you can avoid using towers, do so.
On the second wave, thin out the mobs so there is only one or two to hit the crystal. Either by killing them all by hand and sparing one, or using elemental towers to leave a few immune mobs.
Finish the wave with the crystal below 100 hp. The core will be healed, and you can make defenses as per usual to finish the level.