He and his Skeletons are weak to air troops.

Any combination of units up to 6 Elixir or less will achieve this for a neutral or positive trade.

A good way of countering the Skeleton King's activated ability is by using a combination of a cheap splash and single-target unit, such as a Firecracker combined with a Knight or Mega Minion.

A decently tough splash card such as Valkyrie can also be used.

To counter, simply place a small spell to defeat the Skeletons, and place a ranged or air troop to counter the Skeleton King.

Since friendly troops will count as a soul to him, you can place a swarm such as a Skeleton Army while he is around and punish whenever the army gets countered with a small spell.

For the purposes of determining Elixir trades, the Skeleton King and his activated Ability count as 6 Elixir, as the player must play the 4 Elixir King and then spend a further two elixir on his ability.

The Soul Summoning ability costs 2 Elixir to activate. The ability goes on a 20 second cooldown after the duration ends. The amount of souls the Skeleton King has can be seen in a blue bar above his hitpoint bar. With no souls, the Skeleton King will spawn 6 Skeletons, but with a maximum of 14 souls, he can summon 20 Skeletons.

When a troop dies, the Skeleton King gains a 'soul', which spawns an additional Skeleton when his ability is used. The number of Skeletons spawned is based on how many troops die (either from the player or the opponent) while he is deployed. The Skeletons spawn 1 at a time at random positions in the circle every 0.25 seconds. After a 1 second delay, the Skeleton King activates his 'Soul Summoning', stopping in place momentarily before summoning a varied amount of Skeletons in a 4-tile radius around himself. The Skeleton King has a special ability, accessible from the rightmost side of the screen just above the player's card slots in battle. “Collect souls of the fallen and bring them back from the dead!”

Clash of kings building cost