This is the Kingdra set that I had quite a lot of success with for the Dark Horse Challenge.
Kingdra (M) @ Choice Specs Trait: Swift Swim
EVs: 4 Def / 252 SAtk / 252 Spd
Modest Nature (+SAtk, -Atk)
- Hydro Pump
- Dragon Pulse
- Draco Meteor
- Sleep Talk
It's almost the same as the standard specs set, but I use sleep talk over another coverage move. The purpose of using sleep talk was to have a free switch in to breloom and proceed to immediately ko it. Draco meteor 1hKOs, dragon pulse KOs with SR damage, and hydro pump 2hkos. I imagine it would work well with magnezone support, but I didn't use it on the team I peaked 8th with (I probably should have).

Kingdra (M) @ Choice Specs Trait: Swift Swim
EVs: 4 Def / 252 SAtk / 252 Spd
Modest Nature (+SAtk, -Atk)
- Hydro Pump
- Dragon Pulse
- Draco Meteor
- Sleep Talk
It's almost the same as the standard specs set, but I use sleep talk over another coverage move. The purpose of using sleep talk was to have a free switch in to breloom and proceed to immediately ko it. Draco meteor 1hKOs, dragon pulse KOs with SR damage, and hydro pump 2hkos. I imagine it would work well with magnezone support, but I didn't use it on the team I peaked 8th with (I probably should have).