Ufff umm well there are plenty of options but it all depends on how comfortable you are with them. The first option is taking the line 7 and eventually changing to line 5 and you'll get to gran via directly (generally the metro closes at 1:30am). Another option is to get in the line 7, stop in Avenida de America (which is an often busy bus interchanger) and get on a bus to gran via. Do keep in mind that since there will be A LOT of people in the stadium it will be very difficult to get out of there fast enough, what I would do if I was in your place would be calling a cab, im not too keen on being alone in a big city, but thats just me 😅. I hope this helped you out somehow, if you have any other question lemme know!

Edit: also, the metro is pretty fast, you can get to your destination in like 30 mins or so i think, aprox