"Smoke screen" blocks line of sight of Illusions, so they can't heal anyone. You can deny enemy heals for 5 sec, just with one skill. Right timing and "smoke" placement is key issues.
Try to make him waste his ammo, catching him on an empty mag is like catching him with his pants down. Try to make him panic and use reversal leaving him vulnerable. Don't track him through his dash place your crosshairs at the destination.
If khan picks firing line, he can make all of his teammates in a radius immune to stuns. That renders grumpy bomb and poppy bomb useless. However, this means that you can force him to use battle shout with grumpy bomb.
Fusillade is good against flanks if you can hit the direct shots. Flanks are basicly 2hit kills.
If you cant hit direct shots, just practice! Best way to practice is on Onslaught.