This can be easily done in the first true operation, when you are after Dr. Weir. Before the mission (when you are in the base) hire all 4 available agents, which will give you all the classes. Play the mission as normal until you find the main supply crate, where you can switch your agents. Pick 2 different agents (notice their icons next to their names, those are indicators for their classes), exit from the crate. Enter the crate and pick another 2 classes, as you exit the crate, the chievo will pop!