Instructions
Go to Pinwheel Forrest, which is located at the west end of town. Talk to the first lady on the path and she will give you TM94, which is the Fighting-type move "Rock Smash."
Walk through the grass in the zone. Timburr, Sawk and Throh, which are all Fighting-type Pokemon, appear at random in this area. Sawk and Throh have very low appearance rates, and will only show up in Black or White version respectively. Catch one of the Pokemon and teach Rock Smash to it.
Go to the Nacrene Gym. The Gym is located at the back of the local museum. To get to the gym leader, you have to read clues from a series of bookcases. Start out with the bottom-left bookcase, then the bookcase behind it, followed by the furthest bookcase back on the left, then the bookcase in the front-right and finally the bookcase behind that one. A secret passage opens up below the final bookcase.
Fight the gym leader. Lenora only has two Pokemon, a level 18 Herdier and a level 20 Watchog. Neither are very difficult, but watch out for the move Take Down. It will cause a ton of damage. When you defeat the Herdier, make sure to switch to a low level Pokemon. Watchog sometimes uses a move called Retaliate on the first attack, which does double the damage if used directly after a Pokemon has fainted. It will most likely kill any of your Pokemon in a single move. Once you defeat her Watchog, Lenora will admit defeat and reward you with the "Basic Badge. "