Due to his long attack range, great area damage and the special ability to attack both air and ground units twice, he is a decent replacement for Witch, Wizard, Bowler and Bomber in so many decks.
He kills Skeletons, Spear Goblins and Fire Spirits on the first pass. On the second pass, he can kill Minions, Goblins, Archers, Princess, Ice Spirits etc.
He is excellent against swarms, especially against Goblin Gang, which will be released soon in next 2 weeks. Just keep in mind that his attack speed is very slow (2.4s) so you want to place him far enough from the enemy troops.
Since Executioner has a linear attack, he is pretty similar to Bowler. Place him behind Ice Golem, Giant, Royal Giant, Golem or Hog Rider to make the deadly pushes. On defense, he is great at dealing with the tank all glass-cannons behind it.
Since Zap can’t 1-shot Goblin Barrel anymore, you can use Executioner to take out 3 Goblins in a Barrel (with a precise timing of course).
Mini P.E.K.K.A, Knight and Mega Minion are some great counters to the Executioner. Otherwise, surrounding him with Skeleton Army, Barbarians or Elite Barbarians can easily take him out.