Electro dragon are useful ,if chain lightning only occurs. If you want to block edrag spam attack just protect your X bows, infernos, Air defense, place them at a 2 tile gap from other buildings, if you keep high Hitpoints buildings like storages besides the Air defense with a 2 tile gap from all directions, then you can defend it, and if you keep super minions in cc , then it can even defend without getting 1 star also. Keeping 2 superminions, 1 Head hunter, 1 Ice golem can protect you from queen charge to for 45 housing space.