[Sandman]

I've messed about with this in SP, and I can't see that it could ever be a competitive strategy.

Firstly, assassins are slow, slow, slow. Move, assassinate, move, assassinate instead of just moving forward with a conventional army. You can use tricks like retreat-scouting to make it more efficient, but it's still slow, and the enemy can 'steal' provinces you've cleared of commanders.

Secondly, assassins cannot reliably kill commanders unless they have some serious items. Regular indy commanders can put up a tough fight, whilst mounted commanders are even more dangerous.

There are tricks to help make your assassins more effective. The easiest way is an assassin prophet with a strong bless; he'll be a reliable killing machine. Shrouds of the battle saint work as well.