Purple is not the main enemy, dude. Read the Victory Condition of the scenario properly.
You don't have to confront Purple at all (and moreover, you must not do it according to the in-game texts).
Of course it's Barbarian's Capitol. The in-game texts actually tell you to avoid Necromancers from the very beginning.
Necro town in H2 is absolutely retarded and I question the sanity of anyone who prefers it over the H3 Negropolis.
Thematically they threw everything and the kitchen sink in there. Colonial mansions mixed with medieval (?) skull-castle complete with moat, the pyramid of Khufu and a mage guild that wouldn't be out of place in HoMM3 Inferno. Not pictured is the dragon gazebo that ties everything together. The layout is also completely whack because the empty city is a bunch of fancy cobblestone roads going nowhere (because they didn't change the town background when you upgraded your city hall like in H3). This also highlights the bad perspective some of the buildings are drawn with, but I guess we finally know who ITS hired to draw AoD's world map.
(snip...)
Much as I love Heroes, I never really got good at it or won many maps or campaigns. I tend to lose interest after a while. But I did actually beat Alteris a year ago or so. Although to be honest, I already realised that you only had to defeat the Barbarians when I first played it aged 11 or so. In any case, I don't know if the Necromancers ever really try to hunt you down effectively.