I started recently (normal intensity) so I only just beat Keta. Been swapping out my mons a ton because I feel like using a bunch of different ones but so far, Shuppet, Psyduck, Skwovet, Spearow and Gossifleur have been surprisingly helpful.
Shuppet - is now at the point where it's getting outsped and one shot in important battles but still, will-o-wisp is so helpful against RIFT mons and other important battles, while Pain Split and Cursed Body came in clutch more often than I'd expected
Psyduck - great early game moveset, Confusion makes it really helpful for Venam and Water Pulse lets it pull it's weight in other battles too.
Skwovet - Comes with Bite, gets Stockpile before Venam and Body Slam at level 20. Cheek Pouch + Pecha Berry makes it last even longer against Venam. Also get's Stuffed Cheeks before Venam so pairing that with an Oran Berry could be pretty useful too.
Spearow - Super early Aerial Ace + level 20 evolution will ensure it's one of your heavy hitters for a while.
Gossifleur - I never really liked this line, and I did not think this thing would be good, but damn. You get the Magical Leaf TM at the same time you get it. Leech Seed + Cotton Down makes it super good in important battles, and grass moves are boosted in Sheridan so Eldegoss is even more helpful against Keta (even against Combusken, since Cotton Down can somewhat counter Speed Boost)
Just got an Octillery and a Girafarig on my team, hopefully they don't disappoint. Girafarig is already helpful because Sap Sipper heals it after every turn in the Forest Field and Octillery learns a lot of very good moves via level up.