The Arrow wall trap will unlock after beating the Hallway level for the first time.
Replay the level only using the arrow wall trap and any weapons or spells of your choice.
Space is limited but you won't need more than 2-4 traps as long as you have weapons and spells equipped that can deal with any Orcs that make it past.
The level has two archers which are not removable but also don't impact this achievement.