Finally finished the game, have one nom to add:
Gengar (No Trade) for B/C [depending on how much of an issue its availability is]
Availability: This is far and away Gengar's biggest issue. It can be found in Thrifty Megamart as an SOS encounter from Haunter, which can be very frustrating since Haunter is immune to False Swipe and is frail and has Confuse Ray and Curse. I used a Taunt Gyarados; Salandit is probably best since it gets Dragon Rage early. This is also notably when everything starts getting Dark- and Ghost-type coverage.
Stats: It's a Gengar. OHKO or be OHKOed. Special attacks. etc.
Movepool: When you get it, Gengar notably gets Venoshock, Shadow Ball, Energy Ball, Psychic, and Will-O-Wisp right away and Ghostium Z right after the Ghost Trial, and Sludge Bomb and Toxic are not that far away. It gets Thunder(bolt), Focus Blast, and Dazzling Gleam towards the end of the game, and it can also set up Rain Dance if you want to manipulate rain SOS. Hex at level 55 rounds out its movepool, though it's probably better off sticking with all-out attacking than spreading status.
Major Battles:
Totem Mimikyu: Use something else. You might be able to cheese it with Confuse Ray but there's really no point.
Team Skull: You will struggle with Grimer/Muk and Raticate, but you should outspeed and KO everything else.
Nanu: Never-Ending Nightmare takes care of Sableye and Energy Ball takes out Krokorok. Use something else for Persian.
Team Aether: Muk and Raticate are again issues, and random Dark-type moves fly around everywhere, but you should outspeed everything and KO almost everything.
Lusamine: If you have really terrible speed Mismagius can be an issue. Otherwise, you outspeed and OHKO most of her team (and you're immune to all of Bewear's moves)
Hapu: Her Pokemon will OHKO you, but you can at least snipe Gastrodon (and maybe Mudsdale, I didn't check) with Energy Ball.
Totem Kommo-o: Dazzling Gleam
Hala: All of his Pokemon have random coverage (most of it Dark) so if Psychic doesn't OHKO you're probably not living a hit in return.
Olivia: Energy Ball for days. You could try Focus Blast but I found Energy Ball sufficient so there's no need to risk missing.
Acerola: You outspeed and OHKO everything.
Kahili: Skarmory has Sturdy, Crobat may outspeed you, and you may not be able to OHKO Mandibuzz (Electrium Z really helps here), but Thunderbolt gets you through most if not all of this battle.
Professor Kukui: Ninetales is outsped and OHKOed by Sludge Bomb. Braviary could be a problem if it survives Thunderbolt. Lycanroc can outspeed if you have really terrible speed and OHKO with Crunch, but it gets OHKOed if you outspeed. Magnezone can paralyze you and has Sturdy to bounce back your Focus Blast, so you should probably use something else. Snorlax will likely take a Focus Blast unless you use Fightingium Z and OHKO back with Crunch or High Horsepower. Sludge Bomb probably OHKOes Primarina (I didn't use Litten), Focus Blast with Z-Move OHKOes Incineroar (not sure about non-Z-move), and Shadow Ball probably OHKOes Decidueye (I didn't use Popplio), but watch out for Sucker Punch.
Additional Comments: Gengar has terrible availability for players without access to trade, but the sheer power and flexibility make it easily worth it, especially since it's useful (almost) right away. The ability to switch between so many TMs and Z-Crystals definitely gives it an edge in the late-game. (In fact, I can already see speedrun strats revolving around SOS Gengar) I had a Jolly Gengar and it carried the late-game lol
