Diversion on their Lead attack, if not their offhand attack and they're pretty much screwed.
Spiteful Spirit Apparently this seemingly useless curse is super deadly for the assassins seeing as how they attack much faster than Swords.
And last but not least... Gank.
Same principle that assassins use when taking out a monk.
Just like how AOE is finally coming into play in Arenas when it comes with ritualists.
They depend on their spirits and normally stay near and cluster their spirits.
You drop an AOE (Meteor Shower or Traps) GG ritualists, otherwise someone is going to have to kill their spirits.
With that said, assassins are not overpowered.
Monks can still stay alive if they know what they're doing.
It's just easier to kill a monk now with assassins :)
edit: Fallen - Do not post that pic again, Rizzy