I'll vote for Kyro's set here, but I'd like to slash in Fire Punch over Thunderbolt so as not to be complete Roselia set up fodder. It can also hit Glalie, Tropius, Mawile, and the inasane amount of bug types that litter the tier hard. Explosion over substitute can be an option to go out with a bang. If we use Substitute, Giga Drain would be better than HP Grass because then you can make more subs.
So I propose the set be edited to this:
CCAT FORM
Pokemon: Haunter
Set: Haunter (F) @ Leftovers
Trait: Levitate
EVs: 252 SAtk / 4 SDef / 252 Spd
Timid Nature (+Spd, -Atk)
- Will-O-Wisp
- Substitute/Explosion/Thunderbolt
- Giga Drain/Hidden Power [Grass]
- Fire Punch/Thunderbolt
Play Style: Status spreader, wallbreaker, possible sweeper
Explanation: The RSE NU metagame is mostly ruled by a horde of slow, physical attackers. Haunter is a Fast Will-o-Wisper. Need I say more? Haunter can cripple most of the metagame, which just kinda makes it awesome. This set in particular also has the required coverage to hit the most of the tier (35/55) for SE damage. Giga Drain should be run with substitute only. Sub + Giga Drain for a sub-centered approach, Explosion + HP Grass to take something out nearly every match, and Thunderbolt and HP Grass for best coverage.
Clash: This set is far more diverse, as I mentioned hitting over half the tier SE and the entire tier neutrally. It can explode to get rid of troublesome mons that got a chance to set up, cripple physical attackers, and outright OHKO a good chunk of the tier.