The Electro Dragon is the 13th troop unlocked in the Barracks. They are powerful air units with mediocre health and high damage.
Electro Dragons have no preferred target when attacking; they will simply attack the closest building. However, once they become aware of enemy Clan Castle troops, Heroes, or Skeleton Trap skeletons (either by being attacked themselves or by being near another friendly troop under attack), they will leave their previously targeted building and instead engage the enemy troops. Once all of the nearby enemy troops are defeated, the Electro Dragons will proceed to attack the nearest structure.
The Electro Dragon attacks with chain lightning, capable of hitting up to 4 other targets after the initial hit, with each successive hit inflicting 20% less damage than the previous one. In order for the lightning to chain onto another target, it must be sufficiently close to the previous target.
The targets of the Electro Dragon's chain lightning are determined by distance, with hitpoints as a tiebreaker; to determine the next target, the target that is closest to the previous target and that has not yet been hit is chosen. If the closest targets are equidistant from the previous target, the one with highest (maximum) hitpoints is chosen. Thus, it chains through buildings with most hitpoints near the primary or previous target while ignoring low hitpoint buildings to make full use of its chain damage.
When destroyed, the Electro Dragon causes the nearby ground to be struck randomly with lightning bolts, in a similar fashion to the Lightning Spell prior to its rework in 2020.The Electro Dragon has a short range. However, its high damage and chain ability significantly increases its practical reach. They can act as flying Bowlers (taking more space but with superior health, damage and a different hit mechanic) as they can hit multiple targets at once and help funnel in attacks.
Electro Dragons are very effective against bases with clustered buildings, because it can make full usage of its chain damage.
Seeking Air Mines will significantly damage your Electro Dragons. It is helpful to send in air units (such as Balloons) before the Electro Dragons to trigger any Seeking Air Mines that might ambush your army early. They can also help funnel for the Electro Dragons to enter the core of the base. You can also use a strategy called "Electro Surgery" (from the Practice Mode) by using as many Electro Dragons as possible and filling the rest of the space with Balloons. Be sure to include some Rage, Clone and Haste Spells to support the push.
A Queen Walk can be used with Electro Dragons by using 4-5 Healers to as many Electro Dragons as possible (also called "Electro Queen"). Although the Healers will use space that would otherwise go to 2 or 3 Electro Dragons, this can be offset by using the walk to eliminate weaknesses for Electro Dragons, such as Air Defenses and/or Air Sweepers. If you have any extra space, you can add in some Balloons to tank for some Traps that may be set off.
Heroes (especially the Barbarian King) can tank and funnel for the Electro Dragons, though one should be wary of triggering ground-only Clan Castle troops when using any Hero that isn't the Grand Warden in air mode, as they can distract the Electro Dragons.
The Rage Spell and Electro Dragon make a good combo, as they can greatly amplify its already high damage. They can destroy most buildings in one hit, and even instantly kill Archer Queens alone this way (up to level 71). Note that the spell's effect only applies if the Electro Dragon is within the spell's AOE.For the lightning arc to chain, buildings must be spaced one tile apart at most. Therefore, by spacing them out further it is possible to drastically reduce their effectiveness.
If there are multiple buildings that the chain lightning can jump between, the lightning will chain onto the closest building that it has not yet hit, and if there are multiple candidates for the closest building, the building with the highest initial hitpoints is chosen. Use this to your advantage to redirect lightning away from important buildings; for example, resource storages, despite their non-defensive nature, can be used as "lightning rods" in this fashion due to their high hitpoints.
Clan Castle Electro Dragons on defense are very intimidating troops, as the chain lightning can be used to destroy many group of weaker troops while its high damage devastates tougher ones, unless one has a Poison Spell to damage and delay it.
Electro Dragons are also able to one-shot lower level Archer Queens. This makes the Electro Dragon one of the best defending Clan Castle troops at TH9. It is able to punish rushed or inexperienced attackers. Keeping an Electro Dragon and a Balloon can kill the enemy's queen even with her ability.
Its Chain Lightning allows it to be useful on defense as it can kill up to five spread-out Archers or Wizards at a time. However, its slow fire rate makes it vulnerable to be easily killed by the Archer Queen under her ability; if it is harmed by a Poison Spell at the same time, the Electro Dragon may not even be able to charge up an attack in time before it is killed. The lightning it drops upon death is strong enough to take out entire groups of Wizards and Archers.
If you don't prevent the Electro Dragon's Chain Lightning, it will pick off most of your buildings and even take the Eagle Artillery out without it being able to do much damage or even snipe the Archer Queen.

If you can see a possible Electro Dragon spot that chains all the way to an Air Defense, then use a Hidden Tesla to redirect the shot and also make sure that Air Defense can snipe that possible Electro Dragon Spot to protect it.
The Air Sweeper can push back the Electro Dragon far enough that when it gets back to a building and is about to fire, it can push it back. Basically, it can repeatedly stall the Dragon for a very long time. If you can guess where an attacker will place their Electro Dragon army, then cover it with the Air Sweeper.
A max level Air Sweeper itself can push Electro Dragons far enough that they will never reach it. This could force the use of a Rage Spell, Freeze Spell or Haste Spell that could be used more effectively elsewhere.
Having a Lava Hound could prove useful against an Electro Dragon attack, as its high hitpoints and large number of Lava Pups spawned on death stalls an Electro Dragon attack significantly. However, since the Lava Hound targets ground only, it will not appear unless ground troops (such as your Heroes) are placed in the vicinity.