This achievement is now fixed
"As Chernobog, kill 2 gods after landing and buffed by Living Nightmare."
While the other guide is correct, I thought I'd throw some tactics into the mix. This achievement can be pretty tough.
This is probably best attempted in Arena, it's where I got the award. I'd built specifically to get this achievement, so it isn't a particularly good normal build, but it wasn't too bad for a standard game either way.
I recommend building a mix of attack speed, critical hit chance and physical power. You should be able to cut through most enemy gods, especially squishy gods who haven't built any defence. I had built Warrior Tabi, Odysseus Bow, Wind Demon, Executioner, Deathbringer and Jotunn's Wrath.
Use you ult when low health gods, or squishy gods (most hunter, mages and assassins, depending on their build) are either grouped up away from any guardians or high health/ protection enemies. You'll have to pick your spot to land, and try to make sure you don't land into a trap, i.e. being surrounded by a group of enemy gods.
Once you land, you then have 6 seconds to kill two enemy gods. In these 6 seconds, you gain 30% movement speed, increasing levels of extra damage mitigation (20% at level 1, increasing by 5% with each level). It also applies your passive with every hit for extra damage.
They will have been slowed a little by the effect of your shades, you just need to land the hits and be as accurate as possible. Using a build similar to the one above, you'll be able to hit 500-600 or more crit damage per basic attack (assuming your target has little to no defence items), and you'll be firing pretty fast to top it all off. This will give you your best chance at landing all your basic attacks and getting the double kill.
You can also try teaming up to get this achievement. Pairing with any god who has a hard CC move or ultimate which can pull everyone in together can make it easy for you to hit stunned targets. The best gods for this are Ares (his ult), Cereberus (his ult) and even Baron Samedi (his ult, just don't go for his main target unless they take the main damage the ult gives out and are still alive). You could also try pairing with a gold who has long root or slow, to hold the enemies in place and make it even easier for you. Gods such as Kumbhakarna or Ymir would work. There are many other combinations which could help, but these are probably some of the easier ones.
As stated in the guide above, this seems to be glitched at the moment, as those who unlocked the in-game award already, myself included, haven't had anything pop.
I hope this helps a little!