What troops can counter a Giant Skeleton?

Giant Skeleton, like any other Tank is best countered with swarm.

By far the best swarm in the game is the skeleton army. But I've been countered with, and have been countering with a Witch or Barbarians, the later being quite popular among my enemies.

Pretty much anything that will keep him busy long enough to die, before reaching your towers.

The best strategy I'm aware of is to wait for it to come on your side of the map, and then place the army around him. Even if the enemy has arrows or something similar, the skeleton would'be dead by the time they react.

Good point worth mentioning is, flying swarm is not good, as the skeleton ignores them and keeps going to the tower. It will die eventually but not before dealing 1000+dmg to the tower ...

If you're at or above Pekka's Playhouse then including an Inferno Tower in your deck may help you counter this easily if you're seeing a lot of the Giant Skeleton. Due to its nature this is actually true with any high HP targets, but it works really nicely against the Giant Skeleton because it means you don't have to sacrifice troops to his bomb.

Alternatively, if you can distract him with a low cost building like a Tombstone, attacking him with faster units like minions will allow them to leave the area before the bomb goes off. The Giant Skeleton will run off the path to attack a Tombstone or other unit that is coming at him, so kite him away from your Crown Towers and isolate that bomb!

My main deck uses Giant Skeleton, so I've seen a lot of defensive strategies. To summarize:

• Hordes - Generally Giant Skeleton attacks slow, so he'll only take out one Horde troop at a time, and will get pummeled by the rest. Minion Horde or Skeleton Horde will take him down in seconds.
• Turning - Anything that kills the Giant Skeleton will promptly path right past his bomb. To avoid this, put troops behind him once in your territory, so that they get clear before the bomb goes off.
• Kiting - The best Elixir trade on a Giant Skeleton is to use cheap Horde troops like Skeletons or Goblins to kite him to the center and even across to the other lane. This is most easily attempted when he is not backed up by ranged Area Damage troops like Wizards/Baby Drags (who will instantly splat cheap troops).
• Inferno/Other Towers - Giant Skeleton will target anything, so Towers/Huts are a great distraction, essentially similar to kiting. The Inferno Tower is a positive Elixir trade if he is alone, but does need to build up enough DPS to take him down. If the opponent is using Freeze, Zap, or has Hog/Horde backup watch out for it to be taken down to quickly.

Above all, the biggest priority is to keep the bomb away from your towers and avoid sacrificing too many troops to it.