I'm a huge fan of Ghost types, so I've dedicated most of my playing time to building the best Ghost type team I can. I know my teams not at it's full potential yet, so some kind advice would be greatly appreciated.
This is my singles 3Ds team that I use:
Aegislash @ Weakness Policy
Ability: Stance Change
Max Speed, max attack for a physical sweeper
Adamant Nature
-King’s Shield
-Swords Dance
-Shadow Sneak
-Sacred Sword
Stance Change allows you to have a solid defense when you need it and a great offense when you want it. Swords dance adds +2 attack, and if hit by SE move, Weakness Policy will add +1 sp attack and attack. Shadow Sneak is easy to pick off opponents with, STAB, priority move. Sacred Sword for coverage, and the opponent’s stat changes don't affect the moves damage, so increased defense means nothing. King’s Shield to switch back to Shield Forme, protects user, and if opponent uses a physical attack and it's blocked, their attack is decreased by -2.
Chandelure @ Quick Claw
Ability: Flash Fire
Max sp defense, max hp for coverage
Modest Nature
-Energy Ball
-Flamethrower
-Minimize
-Shadow Ball
Quick Claw is defiantly replaceable, but I like to use it to help minimize. Energy Ball for coverage and chance to decrease opponent’s sp defense. Flamethrower for STAB and has good damage, good accuracy, and a chance to leave a burn. Minimize has +2 evasion per use and can get the opponent pretty annoyed. (Rage quits included) Shadow Ball has STAB and a chance to decrease opponent’s sp defense. A good strategy for this Pokémon is to use Aegislash to draw out a fire type move from your opponent, then switching into Chandelure to boost its fire type attacks. If done correctly, you can predict the fire type attack, allow Chandelure to up its Flamethrower with its ability, Flash Fire, and then use Flamethrower with STAB and Flash Fire to do great damage.
Cofagrigus @ Leftovers
Ability: Mummy
Max Defense, max hp for a wall
Calm Nature
-Protect
-Shadow Ball
-Toxic
-Calm Mind
First use Toxic for the consistent damage. Then use Protect so you can get free healing from the Leftovers. Then move into Calm Mind for the +Sp Defense and the + Sp Attack. Then you jest keep running between Protect and Clam Mind while you gain stats, reduce the opponent's heath, and absorb hits. Eventually you will either gain enough Sp Attack to near one shot an enemy or drain their heath with Toxic, and then you're set for the rest of the battle.
Sableye @ Sablenite
Ability: Prankster
Max hp, max sp defense for a wall
Careful Nature
-Toxic
-Substitute
-Foul Play
-Recover
Prankster plays a huge role in the strategy. Prankster allows status moves to have priority. If you're expecting the opponent to use a status move though, you can always mega evolve Sableye so it will have the ability Magic Bounce to bounce them back at them. Substitute allows you hide behind a sub, Recover to heal yourself, Toxic to deal damage while recovering, and Foul Play to deal good damage to any threats. A good strategy is to either use Substitute or Toxic first, depending on what Pokémon you're facing. If you feel safe, use Substitute. If you feel threatened, use Toxic. Then once the opponent is poisoned, either keeping using Recover to stall the opponent to death, or use Foul Play to deal damage with STAB alongside Toxic. Sableye's first type is Dark type, meaning Ghost type moves are effective rather than super effective and fighting type moves have no effect.
Mimikyu @ Focus Slash
Ability: Disguise
Max attack, max speed for a physical sweeper
Adamant Nature
-Swords Dance
-Shadow Sneak
-Pain Split
-Play Rough
First use Swords Dance to raise attack by +2. Disguise will block the first attack, so that's a free move. Then use Swords Dance again and now you have +4 attack. Either you will survive the next attack or be left with 1 hp due to Focus Slash. You can either use Pain Split to gain a lot of health back, Shadow Sneak has STAB and priority, or Play Rough to do huge damage alongside STAB and the chance to lower the opponent's attack.
Giratina Altered Forme @ Lax Incense
Ability: Pressure
Max speed, max hp
Bold Nature
-Hex
-Substitute
-Will-O-Wisp
-Double Team
I'm still trying to find the perfect setup for Giratina, but here's what I have so far. Substitute gives you a reliable barrier to E moves and NVE moves, Will-O-Wisp leaves burn and lowers attacking power, Hex to assist Will-O-Wisp + STAB, and Double Team is self explanatory. I usually use Substitute first, (depending on the opponent's Pokémon) then Double team about three to four times, then use Will-O-Wisp to set up for Hex and to lower attacking power, then max out my evasion which means I have max evasion + Lax Incense for more evasion, and then use Hex to finish things off. Being in Altered Forme gives a greater boost to its defense and sp defense, so if done correctly you can become a huge nuisance for your opponent.
If you have any suggestions, please feel free to tell me! I also have a lot of other ghost type Pokémon with great strategies, so if you want to know what they're like you just have to ask.