I got Black yesterday. Made it to the first Gym Leader and used Sand Attack 6 times so I could try and level up my Purrloin. His response? Critical hit Bite from Lillipup 4 times in a row, knocking out my team.
:|
Best to try leveling up Pokemon outside of the Gym battles. Although, you only need to keep the Pokemon out for one turn for it to give EXP.
If it can't survive for one turn, let another one of your Pokemon faint so you can choose the Pokemon you want to level to come out without worry of it getting hit, then switch it to your stronger Pokemon.