@nprofile1q... I lived in an upper story south-facing apartment in Mannheim during the 2003 heatwave. It was absolutely awful. I think something like tens of thousands of people died across the continent.
What worked for me was opening all the windows for maybe an hour or two in the morning to get some air exchange, and then closing everything up and lowering the Rolladen for the rest of the day and evening. I'm not sure if rolling shutters are common in the Netherlands, though.
I ended up buying a portable AC unit after that. I got used to working without AC in the heat, but I just needed one room cool enough to sleep in.
Best of luck to everyone trying to survive right now!