I imagine for a set up like this to work you'd have to one shot everything every single wither? Otherwise you'd be dead after aggro? how does it work with a higher level mobs?
Rat mages and archers are 1 wither, hellhounds and silver serpents are 2. Wither casts fast enough that most of the time agro isn't an issue at coon spawn. Add in attunment and gift of renewal and it's kinda hard to die.
The real time saver is in the first to levels though. Essence of wind with a level 6 focus will clear the screen in a single cast. No running around pulling agro or waiting for spawn to clump up.... run to the middle of the screen and cast... everything is dead. Add in tossing EV to kill on the island and up on the ledge.