Fire/Water/Grass - You can use the lol-worthy but still surprisingly effective Heatran/Jellicent/Ferrothorn core. Gyarados makes a decent Jellicent replacement if you want a less bulk and more offense.
Electric - Rotom-Wash or Zapdos if you want more bulk; Thundurus-T or Jolteon if you want to sweep.
Ghost - Jellicent, again, for tanking. Also: Sableye is a good utility Pokemon, Gengar is a decent sweeper with good utility moves, Dusclops and Spiritomb are pretty okay tanks, and Mismagius is a decent bulky attacker (albeit only bulky against special attacks).
Any other type - Do your own research. Find a physical attacker, since almost everybody listed here is a special attacker. --- I claim to be the official Porygon2 of the Pokemon Black 2 board. Scientist MileRun: WaRy associate