This is a team I used to constantly beat Cynthia to grind up other Pokemon's levels with the Exp. Share, and with a couple alterations, it seems to work pretty well for the Champion battle too! I'll provide their movesets, as well as the basic strategy employed to beat Cynthia. Additionally, half the slots for the team are missing, which can be filled out by your starter, or any other strong Pokemon you have to deal with the E4, and act as basic support if either of these Pokemon faint. I suggest the levels for these Pokemon should be somewhere in the 60s.
Their movesets look something along the lines of this:
Abomasnow - Abomasnow can be caught at Mt. Coronet, as well as Route 216, 217, and Acuity Lakefront as a Snover
@ Never-Melt Ice / Icicle Plate / Anything you'd like
Ability: Snow Warning
- Blizzard - Learnt at level 49, always accurate in Hail
- Wood Hammer - Learnt at level 43
- Ice Shard - Learnt at level 15, Abomasnow likely won't have this when caught, so take a trip to the Move Relearner to re-learn the move
- Earthquake - TM26, located in Wayward Cave, requires Strength, Cut, as well as the Bicycle
Azumarill - Marill can be found in the Great Marsh, catch a couple so you can get one with a good nature and Huge Power
@ Mystic Water / Anything you'd like
Ability: Huge Power
- Play Rough - Learnt at level 25
- Waterfall - TM99, located in Sunyshore City, or you could teach it Aqua Tail, which is learnt at level 21
- Brick Break - TM31, located at Oreburgh Gate, requiring Rock Smash and Surf
- Bounce - Learnt at level 15
Staraptor - Starly can be found in several of the early game routes
@ Sharp Beak / Anything you'd like
Ability: Intimidate, or Reckless if you have access to HAs
- Brave Bird - Learnt at level 49
- Close Combat - Learnt upon evolution (Staravia evolves at level 34)
- U-turn - TM89, found at the Department Store for $3000, or at Veilstone, requiring Surf
- Steel Wing - TM47, found at Route 209, requiring Cut. You may replace this with Roost or Double-Edge if you'd like
Now, time for the strategy to beat Cynthia:
Spiritomb - Spiritomb can't do much against Azumarill, so just spam Play Rough until its dead.
Roserade - Staraptor's STAB Brave Bird should be enough to deal with Roserade, but you may also use Abomasnow's Blizzard or Azumarill's Bounce, though as Roserade has super effective STAB for the both of them, I wouldn't advise doing so.
Gastrodon - Abomasnow should be able to simply outspeed and OHKO it with Wood Hammer, just be wary of Gastrodon's Sludge Bomb.
Lucario - Either use Staraptor's Close Combat, Azumarill's Brick Break, or Abomasnow's Earthquake, that should be enough to deal with it.
Milotic - Milotic's may be the most troubling if Garchomp won't give you any problems. Abomasnow's Wood Hammer should be enough to 2HKO it, if Abomasnow is at low health due to Wood Hammers recoil and prior damage, I suggest you use Azumarill instead, as it resists all of Milotic's offensive moves, and can 3-5HKO it with Play Rough, as long as it doesn't spam Recover. If Abomasnow DOES have high health, there's no reason to use Azumarill over it.
Garchomp - Be cautious about this guy, as its Poison Jab can deal a large amount of damage towards Abomasnow and Azumarill. Lead with Abomasnow and use Ice Shard to deactivate Garchomp's Yache Berry, then use Blizzard to OHKO it. If Abomasnow faints in this process, switch out to Azumarill to kill it with your STAB Play Rough. You may want to get additional support for Garchomp, as this team may have trouble with it.
And with that, you should be able to beat her with this team! Also, I'm open to any suggestions you may want to give, such as additional members (as long as they're useful and easily accessible) or moveset changes, feel free to give them!