Give a fair mix of encounters / provide encounters that make sense for the area/enemy, without taking their choices into account. That's the fairest way to do it, though I know you end up feeling kind of bad. So you can skew things a bit in their favor, but they should definitely still have to face encounters they're not well equipped to handle. Might convince them to diversify their capabilities, which is what you really want out of this (certainly don't penalize someone wanting to retrain for the sake of party balance by making it cost money or xp or whatever...I never make retraining cost anything.)

Obviously, enemies who've fought them before and survived, or who have studied them, would know of the imbalance and seek to exploit it as best they can.