The Callidus Assassin...

This thing looks like it could be a beast, taking out an entire unit in a turn. I assume the way to play this is to deep strike it against a dangerous unit, hope to roll a five or six on the polymorphine, then use the neural shredder to clean up the rest of the unit. Has anyone used this with success. Also a rule question, is the neural shredder resolved against the enemy unit's majority leadership like you usually use majority toughness?

first of all. polymorphine still allows the enemy to get coversaves, if they stand in terrain for example.
the neural shredders leadership wounding isnt FAQ'ed/errata'ed yet. just like the necron template weapon.
so id say talk with your enemy about that. i would say it counts the majority.
in addition, the neural shredder only causes double strength instant death on the leadership instead of the toughness. meaning a LD 4 unit can get instant killed.

This assassin was very adept at taking out my vindicare in a game.

You need a good inv save to go against the polymorhine hits.

I ran her Tuesday against some Sisters. She came in turn 3 and took out three sisters from inside some ruins. Turn 4 she charged and wiped them off the table, then ended the game in the ruins getting me linebreaker. She did good but not great. Made her points back though, and like Mordrak it's nice getting to set her pretty much wherever you want.

While there is no faq on it, the screader uses the opp Ld as their T with a S8 weapon, so the rules are that if there is one guy with Ld9 (now T9) and 4 guys with Ld8 (now T8) then it would be T8.
She is a good unit but you will have trouble useing her all the time, you need to watch where you place her and make sure she is supported or she will not last long