So I guess I didn't post a solution to this problem:

The entire point of Melee units is that there is only so much room on the battlefield for them to attack. There needs to be a choke-point so only x number of melee units can attack each round.

If it were up to me the ninja special ability would be to come to the front line and attack without receiving damage in return.

There also needs to be a similar choke for ranged units. Only x ranged units can attack at once. Snipers have a longer range so again they have a separate choke.

I'll talk more about this later with video analysis from a game I play.