Cannot killed spawned Ender Dragon with sword in 1.10.2 [duplicate]

You can use the /kill command to delete the ender dragon, provided you do not use a radius:

/kill @e[type=EnderDragon,c=1]

When you summon an ender dragon with the following command:

/summon EnderDragon

The DragonPhase value defaults to 10, which stands for "hovering". During this phase, its Motion values are locked to "NaN", which prevents you from attacking it with a sword because that relies on its position, which has been corrupted by invalid values. It is for the same reason you cannot use a radius to select it.

If you want to be able to attack the previously-summoned dragon, you will need to both change its DragonPhase value to something else, as well as reset its Motion (which is as simple as providing an empty list):

/entitydata @e[type=EnderDragon,c=1] {DragonPhase:0,Motion:[]}

The dragon will not move from its position until you relog, but you can still hit it.

Alternatively, you can summon it with a different phase, though it will immediately fly to (0,0,0):

/summon EnderDragon ~ ~ ~ {DragonPhase:0}