0093 Haunter

Shadow Claw is one of the best fast moves in PvP, while Astonish is one of the worst. 

Shadow Punch is a low energy charge move that allows Haunter to apply shield pressure and possibly bait shields. A very poor 1.14 DPE means it won't be dealing as much damage, but can still sting quite a bit thanks to Haunter's high ATK stat along with STAB. Shadow Ball gives Haunter the extra power it needs to do deal massive damage and possibly KO most opponents. Dark Pulse is an inferior move and lacks STAB. Sludge Bomb is also a viable pick now partly due to STAB coverage against an increasing popularity of Fairy-types. Ice Punch is an alternative option that lacks STAB but can be Super Effective against various meta Pokemon. 

Gengar is known as a glass cannon, and Haunter isn't much different. The obvious lack of bulk causes Haunter to lose matchups that it can't quickly finish, and also requires Haunter to demand more shields to preserve its already limited durability. Haunter somewhat makes up for this by offering a very high ATK stat which allows it to punish its victims faster, though this differs from the more typical strategy of aiming for bulk due to the CP formula inflating ATK stats. Just to put this in perspective, an unshielded Shadow Ball deals roughly 42% of Bastiodon's max HP, and that doesn't even apply a Super Effective multiplier. Utilizing Haunter in actual fights is rather difficult and requires a lot of experience and practice, as its fragility and Poison sub-typing can be severe liabilities in its role as a Ghost-type Pokemon. 

With a max CP of 1963 and low bulk Haunter is mostly restricted to Great League. If you want a Ghost-type Pokemon, Giratina is king here. 

Attack fast, faint faster. Use Giratina instead.