Yep, my assassin is working her way towards 94 at the moment. Act 5 Hell runs are mostly pretty easy for her, and thats where I've done most of my solo levelling.
In cows, the biggest risk is getting swarmed. I recommend lots of DR gear, and lots of leech seems to help. Death Sentry rocks, particularly if you have other people to soften up the crowd (with Static Field for instance).
The Tiger Strike/Dragon Tail assassin is supposed to be good in cows but I've never tested it. My assassin uses Phoenix Strike as her main attack. The PS Freeze Charges can stop a mob in its tracks, and Mind Blast is also a life saver (just be careful of it when your party has Might or Fanatacism active...).
Chris