How from Serbia to Barcelona?

If you looking for the cheapest way to travel overall, then probably using low-cost flights will be least expensive (for instance RyanAir or WizzAir from Budapest or Vueling from Zagreb, or maybe even Venice) and obviously faster, since most of the route you'll fly. But in case you interested in traveling by land only, then it will be a bit more complicated, mainly because of number of changes and a bunch of tickets, although still possible. For example, Belgrade-Ljubljana with 25 euro discounted ticket, then Ljubljana-Nova-Gorica another roughly 9-10 euro, next Gorizia-Ventimiglia will be at least 30 euro (and that's basically the best you can get with in advance purchase), then a few euro over the border and roughly another 60 euro till French-Spanish border (Cerbere), plus another 15 from there to Bacrelona (or direct Elipsos Milan-Barcelona also can be used). Or alternatively to Budapest, then Budapest-Zurich and again Elipsos to Barcelona etc.
Also possible to opt for half by land-half by air routes, something like Serbia-Norther Italy by land and then fly. Or alternatively even fly out of Serbia with a change somewhere etc. There are a lot of travel criteria (number of changes, travel time, price etc.) which you need to account and decide which are of primary importance for you