Most narrative games I find are written fairly poorly.
A couple ideas on how to balance out the sustained assault rule
Prepared positions: The defenders knew this day was coming. While they did not know when, they have been busy laying traps and preparing for it. All terrain pieces out side the defenders deployment zone may have booby traps. Every time an enemy unit enters or leaves a terrain piece, roll a die. On a 1, nothing happens, on a 2-5 that unit takes d3 mortal wounds, on a 6 they take d3+3.
Fire for Effect! The defensive guns don't need to see - they just need to keep shooting. With that many targets, they are sure to hit something... For every infantry unit not in the defenders deployment zone, roll a d6 at the start of the defenders turn. On a 4+, that unit takes d3 mortal wounds
Cut off the head! An army is only as good as it's leader. Cut of his head, and you cut off its. The attacker looses the Sustained Assault rule if their warlord is slain
Rise to the challenge! When the going gets tough, the tough buckle in and hold on tight. Every turn in which the Attacker brings on units through sustained assault, the defender gains an extra CP. If the Attacker brings 3 or more units on, the defenders gains d3 CP instead