I've managed to beat hard, you have the right idea with the smoke grenades. Over the coming rounds leading to round 10 pile up a bunch of smoke grenades in the center of the map and on the round time between the switching of waves 9-10 toss them into all spawn locations and pray. This works sometimes because as the waves go on the amount of time it takes for the infecteds to spawn in goes down exponentially. If you happen to miss some infecteds and you're getting piled on, run towards a rapidly inflating infected and sandwich it and you against a wall to trigger the game to spawn you in a set location as a failsafe to get you unstuck, this can help or hurt you, so make sure there are no infecteds in that area before you preform this. If done correctly it can give you some breathing room and get you away from a horde.
Best of luck!