Archimonde was the big-bad-guy at the end of Warcraft III: Reign of Chaos' campaign. In that game, Archimonde was responsible not only for ruling over the Scourge and the Burning Legion, but also for single-handedly destroying the city of Dalaran. In the final mission of Warcraft III: Reign of Chaos, players had to stop this powerhouse from reaching the World Tree. However, Archimonde would not make this an easy feat. Despite the Orcs, Night Elves, and Humans working together to stop him, Archimonde simply walked right through each of their bases, killing anyone in his way.
In Heroes of the Storm, Archimonde would undoubtedly act as a warrior-class hero, with a touch of specialist antics. Dark Portal could be used to summon demons in the same fashion as Azmodan. Finger of Death would be his high-damage single-target spell, and he would also have a passive bash in his arsenal.
Rain of Chaos would act as his heroic ability, summoning multiple infernals that would come crashing down for initial splash damage before running about and destroying enemy forces. That, or maybe he could make a sand castle like he did in Warcraft III before smashing it and watching an enemy building collapse.