End Crystal Destroyer

EDIT: Don't use obsidian as it doesn't protect the redstone. However if you build it high enough the dragon wont damage it. Don't build it too high either because the TNT will explode in mid air.
Yesterday during the Hermitcraft livestream, I saw a suggestion in the twitch chat for destroying the End Crystals with TNT and I decided giving it a go.
So I built a large rectangle of redstone protected with obsidian (so that the dragon doesn't destroy it) and used dispensers to drop TNT on top of the crystals.
This way you can easily kill the dragon multiple times.
Things to consider:

• Snowballs don't work beacuse some crystals spawn with an iron cage
  
• Dispensers should be placed on the side of the towers because anything above them gets destroyed when the crystals respawn
  
• Dispensers should be placed high enough so that they don't get damaged by the explosion
  

Materials:

• Arround 8 stacks of obsidian
  
• Arround 4 stacks of a building block
  
• Arround 4 stacks of redstone
  
• A few repeaters
  
• A few dispensers