I Know the wall of the defending army needs to be low to get the attacker berserker to hit with full power.
This is incorrect. All that factors in is provisions. Berserkers need to be outnumbered at least two to one to get their special effect. Obviously given the resources and time it takes to build zerks, you don't want to throw one at a high wall, but the wall itself does not matter when calcing the zerks damage.
If the defender has two times the spears you have zerks, they will not proc. If the defender has twice the HC, they will proc (HC and zerks equal provs). You would need twelve times the spear to zerk ratio. It's about provisons.