I am not contesting what you say, more factual description is that Germans fought against your grandfather who was in Tito's partisans.
Even according to official Communist propaganda of Tito's time, all major partisan battles with Germans (Uzice,Igman, Kozara, Neretva, Sutjeska, Drvar) were called 7 ENEMY offensives.
There was not a single PARTISAN OFFENSIVE against Germans.
It was a war on German initiative. No wonder, the principal objective of the Partisans was communist revolution, not liberation of the country.
As a matter of fact, Communist Party became interested in fighting Germans only AFTER Hitler's attack on USSR in June 1941. Until then, Communists were friendly with German occupying force while Gen. Mihailovich forces fought against Germans.
Another telltale: Communist propaganda called attack on USSR "Treacherous".
Partisans were guerilla wariros, lightly armed and with no classic military training.
Nowhere in Europe partisans/guerillas ever fought offensive actions against germans.
Up until Sremski front in 1945.
But just to tell you about how partisans fought germans, attack on drvar in late 1944 were Nazis tried to capture Tito. Partisans fought nazi paratroopers so hard that almost entire 500th SS parra battalion was whipped out.