If you were to use this method to farm items, it will be an inefficient way.This needs to be a thing.
Except ONE note: mobs preaggroed on you shouldn't just turn around and attack something else. If you got chased by a Unicorn all the way back to the Corruption it shouldn't just see a Face monster and go after that instead. (Not for logic but for balance, otherwise I'd just stand in the divide between the two biomes and shoot everything.)
As for the mobs, they lock on to a mob (Bulldog mode) and switch targets only once the mob it was targeting is dead or it has reached a critical health level.