Pro tip: if you want to spec a spy in the skill of slowing armies down, it is way better to spec in assassination instead. Not only you get rid of an enemy general (and have the potential to get rid of enemy agents as well) but killing them also stops the enemy army for an entire turn because they have to recruit a new general, spawning with no movement range of course. Effectively it is 100% movement reduction sabotage supplies skill with the addition of getting rid of an enemy general. It's a no brainer.

My question for discussion: what could be some improvements to the sabotage supplies skill to bring it up on par with assassination?