@ Leftovers
Trait: Water Absorb
EVs: 252 HP / 36 Def / 220 SpD
Calm Nature
-Scald
-Recover
-Toxic/Will-o-Wisp
-Shadow Ball
This pokemon just cockblocks all standard keldeo sets. Immune to both of its STABs and resists it's 2 most common coverage moves. HP ice/Ice Wind and HP bug. I've never had a better pokemon to check Keldeo. Between Toxic/WoW and Shadow ball, it punishes keldeo for every time it locks itself into one of it's STAB/coverage moves.
HP ghost is the only thing keldeo can run hoping to at least do some respectable damage to it.
But that fucks over common and well accepted best "Keldeo counters" like Slowking which imho is rather situational as it gets 2HKOed after rocks and a chance 2HKO even without rocks.
Okay lets just assume the worst case scenario.
Scarf Set
252 SpA Keldeo Hidden Power Ghost vs. 252 HP / 220+ SpD Jellicent: 110-130 (27.22 - 32.17%) -- possible 4HKO
Specs/Expert Belt
252 SpA Choice Specs Keldeo Hidden Power Ghost vs. 252 HP / 220+ SpD Jellicent: 164-194 (40.59 - 48.01%) -- guaranteed 3HKO
Calm Mind Set
(assuming you're a pretty shitty player and let something like Keldeo even get to +2/+2)
+2 252 SpA Keldeo Hidden Power Ghost vs. 252 HP / 220+ SpD Jellicent: 216-256 (53.46 - 63.36%) -- guaranteed 2HKO
STILL not an OHKO, enough to break sub/toxic and revengekill. But IMHO you shouldn't let keldeo get more than +1 anyways.