I think they are pretty easy to get ahold of however the trick is getting the right Nature for one and you have to lose against Team Plasma in order to find out which Nature your Reshiram or Zekrom has otherwise you'll end up beating Team Plasma without even knowing and you'd end up getting a crappy Nature for the Pokemon you have.
It happened to me cause it wouldn't let me check, one time I caught a Victini with a Modest Nature when I should've gotten a Timid Nature instead when I realized that while I was EV Training it I had difficulty getting it's Speed up to 300+ without the EV Reducing Berries which I don't know If they are available in the Dream World yet or not.
It happened to me cause it wouldn't let me check, one time I caught a Victini with a Modest Nature when I should've gotten a Timid Nature instead when I realized that while I was EV Training it I had difficulty getting it's Speed up to 300+ without the EV Reducing Berries which I don't know If they are available in the Dream World yet or not.