The spawned Skeletons are very good at distracting high damage, slow hit speed enemy units, such as the P.E.K.K.A., the Prince, the Mini P.E.K.K.A., and the Giant Skeleton. This is especially effective against units that target buildings, such as the Giant and Golem — the Skeletons spawned after the Tombstone's destruction can damage, if not destroy, the unit.
It can also distract other units for a while, such as the Princess or Mortar.
If a Royal Giant is locked onto a tower, the player can use the Tombstone to push the Royal Giant so it retargets to the Tombstone. This cannot be done with any defensive building as they do not have a large enough hitbox to push the Royal Giant but the Tombstone works because the spawned Skeletons have enough hitbox space to push the Royal Giant. The skeletons will then chip away at the Royal Giant's health.
For more effectiveness, the Tombstone can be placed near the center of the player's territory so that the troops are drawn further away from the Crown Tower. They will be distracted by the Skeletons and the building and will be unable to target the Crown Tower. Also, both of the Crown Towers will be able to attack the troops, assisting in their defeat.
The Tombstone can spawn up to 18 Skeletons, 14 of which are spawned normally over the lifetime of the Tombstone. It then spawns 4 more when it is destroyed. Considering that a Skeleton Army costing the same amount of Elixir spawns 14 Skeletons in total, playing a Tombstone would profit 4 Skeletons, but note that they don't spawn all at once!
The Tombstone is rather ineffective against Bowlers and Executioners, since their projectiles have the ability to not only destroy the building, but immediately destroy the the recently spawned Skeletons.
Do not place this card in a lane offensively without any support since the Skeletons it spawns get destroyed by Crown Towers in 1 shot and do not function well as cover troops.
The Log can be an excellent counter to the Tombstone. If timed correctly, it can not only destroy the Tombstone but also destroy the Skeletons that spawn upon its destruction.
If you do not have the Log, Arrows work as a good substitute as they do roughly the same damage, however it is more expensive and will not destroy the Skeletons spawned upon its destruction.
Another alternative to the Log is the Fireball which can instantly destroy the Tombstone. However, this is a negative Elixir trade of 1 unless other troops or buildings are also damaged. Like Arrows, the Fireball will not destroy the Skeletons that are spawned upon its destruction.
The Tombstone card was added with Clash Royale's soft launch on 4/1/16.