In Pokemon Black and White, the Elite Four can be battled in any order. The room to the southeast is the room of Marshal, the Fighting Elite Four Member. Use Psychic-type and Flying-type Pokemon to take his team down.
In your first battle with Marshal, he has a lv48 Throh, a lv48 Sawk, a lv48 Conkeldurr, and a lv50 Mienshao. The recommended level for your Pokemon to be at is lv53. His Throh has Stone Edge, Payback, Bulldoze, and Storm Throw. His Sawk has Stone Edge, Karate Chop, Grass Knot, and Retaliate. His Conkeldurr has Stone Edge, Retaliate, Grass Knot, and Hammer Arm. And finally, his Mienshao has U-Turn, Jump Kick, Rock Slide, and Retaliate.
After you receive the National Dex, you can challenge the Elite Four again, as well as Alder. His rematch team is: Breloom lv71, Throh 71, Sawk lv71, Mienshao lv71, Toxicroak lv71, and Conkeldurr lv73. The recommended level for your Pokemon to be at is lv78.
His Breloom has Spore, Grass Knot, Sky Uppercut, and Mach Punch. His Throh has Earthquake, Retaliate, Grass Knot, and Superpower. His Sawk has Close Combat, Stone Edge, Poison Jab, and Retaliate. His Mienshao has U-Turn, Fake Out, Hi Jump Kick, and Acrobatics. His Toxicroak has Earthquake, Toxic, Venoshock, and Low Sweep. And finally, his Conkeldurr has Hammer Arm, Stone Edge, Earthquake, and Payback.