Simply start a campaign with any of the races from the Old World and win the campaign. Difficulty does not matter and you can win the short campaign, no need for the long one. I found the Empire rather easy and "fast" (it will still take some time).
To win you will need to:
Hold most of the Empire - you can confederate most later on easily, though vassalage and military alliances are possible as well.
Beat the Vampires to your West - Vampire lack long range attacks and use lots of morale damage, big monsters and large infantry. I recommend some artillery, halbediers and Demigryphs.
Have Archaon be KOed - can even happen through AI factions, but by the end he should be easy enough to defeat
Have most capitals either in you posession or in a military alliance - with the Empire you will have good relations with High Elves, Dwarfs, Bretonnia and even the Wood Elves easily enough. I conquered the free states in the south and Kislev in the north to get what I couldn't through diplomacy
And lastly: no non-chaoswastes-region may be held by a chaos faction, which you should get done by doing the others anyway.