I just reached Rank A today, I can share my team and a few tips below :
Garchomp @ Soft Sand
EVs : 252 HP / 252 Attack / 4 Defense
Adamant Nature
- Dragon Rush
- Earthquake
- Dig
- Swords Dance
Garchomp was my lead. The main idea here is to scout around to see the other players' Pokemon. If you feel like you have a shot, pop a Swords Dance, then Dig. Once you get out of Dig, you should be right next to your target, able to throw a big Earthquake (or Dragon Rush) and get an easy kill.
Ampharos @ Focus Band
EVs : 252 Sp. Atk / 252 Speed / 4 Defense
Timid Nature
- Thunderbolt
- Dazzling Gleam
- Light Screen
- Reflect
Ampharos is quite a nice "Anti-Gyarados" and "Anti-Mega Feraligatr". It's a bit fragile though, hence the Focus Band. Apart from killing Water-types, I mainly used it to pop down the Screens before switching out to another Pokemon.
Skarmory @ Skarmorite
EVs: 252 Attack / 252 Speed / 4 Defense
Jolly Nature
- Fly
- Steel Wing
- Drill Run
- Swords Dance
Mega-Skarmory is OP. You may assume that Skarmoy is a Defensive Pokemon, but Mega Skarmory has like a base Attack of 140. Plus, a ton of resists like regular Skarmory. When protected by Ampharos' Screens, that thing is almost unkillable. Just pop a Swords Dance and Fly. Steel Wing can oneshot most Fairy-types, which is quite nice (technically, "Iron Head" does more damage... but the range is terrible. Steel Wing is a bit more reliable, imo).
Good luck if you're grinding for rank A ! Hope that helped