Well, unfortunately, Yugoslavia didn't get rid of the Nazis all by its own. If that would be feasible, then we wouldn't let the enemy take the country at the first place, right? Red Army had very much to do with it. I mean, if we look Serbia as a part of Yugoslavia (as it was), in three years of fighting, resistant movement haven't done anything significant except causing huge retributions by the Nazis. And then Soviets arrived and drove Germans out of Serbia in just three weeks, liberating even major cities, like Belgrade.
I think that resistant movements through occupied Europe varied in their form of resistance. AFAIK, in theory partisans should keep low profile as long as frontline isn't near enough that they can wreak havoc and maybe even cause front to collapse on some parts. This is what Polish patriots tried to do with Warsaw uprising, only to find out that Soviets were much more cynical that they could ever imagine. In France, La Resistance did kill some of collaborationists, performed espionage missions and sabotages. In Yugoslavia, from various reasons, things looked quite different. I think that all this can be modeled in the game very accurately.
Also, Yugoslav gov. in exile had supported one resistant movement, and then toward the end of the war, they'd proclaimed that now they are supporting the other one. The first one did very little, if anything vs. Nazis compared with Tito's partisans. Also, they did some war crimes and atrocities, and gov. in exile agreed, pressed by the UK, to switch sides. So it's rather complicated, but could be done using rebels with different cause. Tito's partisans did tie a lot of German divisions in Yugoslavia, but the price was very high. At the end, Yugoslavia's population loss was over a million people. Compare it with France, or even UK… So, if we speak in HoI2 terms, I think that partisans were capable of winning battles against garrison units, but not against regulars, before Red Army had come. After that, we are not speaking about partisans (or guerrilla) anymore, as they joined the Soviets in the classic frontline warfare.