Suicune @Leftovers
Pressure
252 HP/128 SpA/126 Def
Relaxed nature (+Def, -Spe)
-Calm Mind
-Toxic
-HP Electric
-Substitute/Rest
Yeah, mono Electric damage sucks, but it stops Suicune being Taunt bait.
I find this fairly unorthodox, but I've used it before to counter Keldeo and I've found it can do quite a good job of it. It can switch in on Hydro Pump and HP Ice, and its huge defense means that Secret Sword will takea few hits to KO it. Meanwhile, it can set up a sub (if using), Toxic Keldeo and set up CMs. If HP gets low then Rest (if using). Eventually due to PP issues from Pressure will mean Keldeo will have to resort to use HP Ice and Hydro Pump. Eventually due to being Toxic'd and being hit with HP Electric Keldeo will find itself low on HP.
EDIT: Changed Extrasensory to HP Electric.