Atlas is actually a great counter to Khan, as he can setback khan after battle shout, has a better shield and can rewind after being thrown out of the map. The main thing is to time your abilities correctly.
It is crucial to avoid her obliteration damage, as she is immune while doing it, deals plenty of damage and will even heal her if she has saved darkness stacks.
If you believe you are gonna have a hard time against Koga, go Power Cosmeum and grip him every time he is near you, having a teammate to kill him while crippled.