[Cohen]

I agree with you saying Assassins should start near the victim and have the first strike.

However you can Improve them ...

For example with Abysya, don't research the first turn, and forge a fire sword. This will make the assassin far more better.
Later you can add more magical items.

Cuthzrael is powerful, but equip him to now waste him for unlucky.

Another way to enhance them is to prophetize. But you can have only a lone prophet ... (Cuthzrael prophet very nice).