Since no one has done this one yet...
Cubone, Marowak
Tier: C or D
Availability: Mid-late (byxbision wasteland)
Stats: Good
Movepool: Meh
Comments: Are you mad at losing against Aya? Does your team not have enough coverage against her poison types? Have no fear, Marowak is here! Currently, this thing holds the highest attack stat any pokemon can reach in the whole game (ep. 11). Assuming it has perfect IVs, an attack-boosting nature, max EVs and you give it a Thick Club to hold, it can reach 400 base attack at level 70 (current level cap iirc). Pair this humongous attack stat up with its signature move, Bonemerang, which is essentially a 90% accuracy Earthquake that can break through substitutes and only hits one target, and you will be able to OHKO (in most scenarios) anything that doesn't resist it. Sadly though, like most pokemons in this game, Marowak has its drawbacks: its special defense is only average and it's incredibly slow, meaning you'll most likely take a hit before even moving. Its typing is also not that great defensively, since it's weak Water, Ice and Grass-type moves, two of which are pretty common offensively. Not to mention the fact that the best moves it can use consist of Bonemerang and Double-Edge (+Rock Head). You could also breed to get one with Iron Head, which allows it to become a great fairy killer. All in all, I'd definitely recommend using Marowak. Again, it's a great mon if you're stuck in Aya, and it also comes in very handy in the following events (bar Serra). All you need is patience to find one holding a Thick Club, raising it shouldn't be that hard.