my friend has a khorne bloodbound army and always fields a slaughter- priest. the problem i have with him is that his ability blood boil always guarantees him D6 mortal wounds. this means that he has to camp 16 inches behind his lines and absolutely wrecks my troops. on top of that he always hides behind cover and is unreachable to my arty. this guarantees him wins during siege battles and makes my life super hard.
Shadowstrike with 6 ripperdactyls is a hard counter to any single model. Unless you are unlucky with rolls, they will eat any character in one combat phase. The only problem with it good screening with multiple models. But this also means that a unit of two has to be left to guard a character and it won't participate in battle. Large units also harder to hide, unlike a single model, so they can be taken out from a long range.
You could also use Terradons in a Shadowstrike, if you fear that you might not get into melee due to screening. And then of course you could use Chameleon Skinks. That Slaughterpriest has a 5+ save I think, so 10 Chameleons might just one-shot him. (they do ~8 wounds on average in their shooting phase, and in a pinch you might set them up right next to him and also go into melee. Remember that they have a great save when in cover so they might actually survive a few attacks as well) Another way could be to teleport a shooting unit like a Bastiladon or Razordons behind enemy lines using LoSaT and shoot him. His save is so crappy, he will hardly survive a round of shooting (the Basti even has rend), and his pals can only charge the unit. Razordons might shoot the units charging them, and even if they die they are pretty cheap, and the Bastiladon will probably survive the attack and you can teleport him out if necessary. Either way it forces the enemy to charge into the wrong direction, that might help you with killing their front lines. And then of course you might be able to kill him using endless spells. Oh and btw: It doesn't. It is is a 4+ chance isn't it? And if he has a 1 he even gets d3 wounds himself.
I think chameleons and ripperdactyls in a shadow are your two best options and are even better paired together. You will use the ripperdactyls to clear any screen and hopefully make him lose look out sir. Then based in my math unit of 10 chameleons should be able to a average 5-7 wounds without the double damage if you select him as target for the shadowstrike and he doesnt have look out sir. And they can do 4-5 if he still has lookout sir.
You could always put Lord Kroak on a Balewind Vortx with a Saurus Astrolith Bearer and repeatedly target the Slaughter Priest with your mortal storm spells.