An effective strategy is to place a high hitpoint unit, such as the Giant, and deploy the Graveyard near the Arena Tower. The Giant will tank the damage while the Skeletons destroy the tower.
However, the strategy can be easily defeated by a combo of Valkyrie and Mini P.E.K.K.A. for a positive Elixir trade, with the Valkyrie destroying the Skeletons and the Mini P.E.K.K.A. destroying the Giant.
Since Graveyard can be placed anywhere on the map, it has great synergy with the Miner, as the Miner can tank for the skeletons as they decimate the tower. However, this combo is weak to the Valkyrie as she deals splash damage, and the Minion Horde as the skeletons, nor the Miner can attack air units.
This problem can be temporarily solved with a Tornado, as it can pull the units away from the Graveyard, allowing the skeletons to continue dealing damage. Also, if paired with a Zap or collateral damage from a Princess, it can completely destroy a Minion Horde.
A Skeleton Army can be a great counter to the Graveyard for a 2 Elixir advantage. As the Skeleton Army covers a large area, they can quickly react to the Skeletons that are newly spawned and destroy them before they have a chance to do any damage.
The Skeleton Army is susceptible to Zap and Arrows, however.
If no good counter cards are available, Zap can be used to kill several grouped Skeletons. If the Zap is used too early, then the rest of the Skeletons will continue to attack the Arena Tower.
Units with large amounts of hitpoints such as the Giant and Golem can be used to take the damage from the spawned Skeletons from the Graveyard.
A possible counter to the Graveyard is the Poison, sharing the same duration, with the Poison covering most of the radius of the Graveyard; however, some skeletons will still do damage to the Arena Tower as the Poison's continuous damage has a tick of 1 second, for a net cost of 1 Elixir.
However, it is possible that the opponent may send in spawners or big pushes as an advantage.
The Graveyard can be a major counter to the Inferno Tower, Inferno Dragon, and Sparky. The spawned Skeletons can easily distract the troops for a minimum of 10 seconds while the defending troops destroy the attacking units.
Pairing this card with the Freeze will allow the spawned Skeletons to survive longer and allow them to deal a lot more damage.
Graveyard, Freeze, and Rage together is a high-risk, high-reward combo, costing a heavy 11 Elixir. Place down Graveyard, then cast the Rage, and once they put something to defend, place down the Freeze. If done correctly, this is able to take down a Crown Tower completely.
However, the opponent can place other cards after the Freeze is cast, or make a push at the other side.
If your opponent does not react to the Graveyard, it is able to quickly rack up massive damage to an Arena Tower, most effectively used after making a positive Elixir trade.
Do not place this spell in the center of a targeted Arena Tower as there is a 9/10 chance for a Skeleton to spawn near the King's Tower and activate it early. Instead, place it on the side of the Arena Tower that is furthest away from the King's Tower to prevent any Skeletons from spawning near it.
The Graveyard can be used as an adequate substitute to the Goblin Barrel.
The upsides of the Graveyard is that it spawns more troops than the Goblin Barrel and is less predictable, while the downsides are that it is more expensive, harder to obtain/upgrade, and is more risky.
Due to its 2.5s spawn time of the first skeleton (1s server delay + 1.5s deploy time), it is not a good reactive card and may be required to be placed beforehand.
A way to use this card for a rush is to place the Hog Rider as a distraction while placing the Graveyard on the side of the Arena Tower you want to attack, usually combined with the Rage.