Serperior @ Normalium Z
Ability: Contrary
EVs: 8 HP / 248 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Leaf Storm
- Hidden Power [Fire]
- Hyper Beam
- Taunt

Keldeo @ Choice Scarf
Ability: Justified
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Scald
- Hydro Pump
- Secret Sword
- Endeavor / Focus Blast / Icy Wind

Scarf Keldeo is a really good cleaner that has been gaining popularity recently, but it struggles to break through Pokemon such as Amoonguss, Mega Venusaur, and AV Tangrowth. Normalium Z Serperior is a really effective lure to those Pokemon, with +2 Breakneck Blitz severely weakening them.

+2 248 SpA Serperior Breakneck Blitz (200 BP) vs. 232 HP / 80 SpD Venusaur-Mega: 240-283 (66.8 - 78.8%) -- guaranteed 2HKO
(kills with Rocks + Leaf Storm)
+2 248 SpA Serperior Breakneck Blitz (200 BP) vs. 248 HP / 216+ SpD Amoonguss: 259-305 (60 - 70.7%) -- guaranteed 2HKO after Black Sludge recovery (very close to killing w/ SR + Leaf Storm, use something like Pursuit to revenge it)
+2 248 SpA Serperior Breakneck Blitz (200 BP) vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 217-256 (53.7 - 63.3%) -- guaranteed 2HKO (Kills With Leaf Storm + a little chip and sr)

After these Pokemon are weakened, Keldeo can clean lategame. Taunt Serperior also helps Keldeo with its matchup vs. bulkier teams, which Keldeo has a bad matchup against. Keldeo can also check Fire-types for Serperior, and Serperior gets a free switch on the Bulky Water-types that Keldeo hates.

edit: edited the calcs, thanks. nature power is inferior because of the less popularity of lele but if lele is used on your team, its definitely an option. I haven't tried it and I think that hyper beam is generally superior.