Unupgraded poison/freeze does NOT work. I repeat, it does NOT work. I tried it in the trainer, and it sucked so much, I only won about 1 fight out of 4.
I find I'm winning about half, actually, against t50. Having said that, I do have an upped Arsenic which helps. To be fair, if you're t50 grinding, losing to everything except the rare farms isn't a problem because the point is not to get electrum, it's to win rares.
EDIT: With a couple of Physalias and a bit more practice, I'm winning a good 80%+. Losses tend to be either against mono-fire, or when a Rainbow deck establishes board control a lot faster than normal.