Turric4n said:
how can you knockback a ghost?
You don't knockback the ghost, you knockback the two heroes that can hurt the ghost. If the melee guy with the hammer and two ranged attackers are in the central section of the dungeon (with the ranged attackers in the bottom two squares of it), there are no trees that the Aria can hide in that the hammer hero can't use his three squares of knockback to get his friends within fatigue distance of battling her.