I did most of the hardcore achievements in Adventure Mode and never bothered with story. However, I cannot find Iskatu boss there. I've noticed that in Act III there is cursed chest which spawns Ghom. Is there some similar event to summon Iskatu? Is there any other way to fight him without doing story mode?
Nope. Iskatu is story mode only. Some bosses and events were taken out of Adventure Mode when it was first introduced and the Iskatu encounter has never been put back in.