plenty of experience with this Keld set.
Zygarde @ Leftovers
Ability: Aura Break
EVs: 248 HP / 124 SpD / 136 Spe
Careful Nature
- Thousand Arrows
- Substitute
- Coil
- Toxic
Since Keldeo lures in things like Fini / Tang / Bulu (all counter Zygarde) we should use something that's takes advantage of that such as Zygarde.
This duo also eases the match up with stall tremendously as you'll find plenty of set up oppurturintes with them having to worry about CM Keld and Sub Zygarde. Although I would have preferred it to be sub cm so we actual beat Toxapex but alas.
Keldeo also beat all Lando sets 1v1 giving Zygarde a much easier time in setting up.
BaSr gave me the EV spread but I'm sure Martin will critique it. Giga Drain from Mega Venusaur never breaks sub. (max spatk modest needs a roll to break.) Letting you get a coil. I think you outspeed the regular Sub Coil spread but don't take my word for it.
0 SpA Venusaur Giga Drain vs. 248 HP / 124+ SpD Zygarde: 67-81 (15.9 - 19.3%) -- possible 6HKO
252+ SpA Venusaur Giga Drain vs. 248 HP / 124+ SpD Zygarde: 94-112 (22.4 - 26.7%) -- 31% chance to 4HKO