You will want your Pokemon to be at least level 80 of you either want to re-battle the elite four: level 70-80, battle Cynthia: level 70-80, or challenge the black tower: level 50- 86. Cynthia is in Undella town in the 1st house to the right of the Pokemon center and the black tower is in black city.
You can also battle people that are level 60-70 in the big stadium and small court in nimbasa city. Once you beat the elite 4 for the first time, you can battle alder a the training facility in Flocessy Town. He will have his old Pokemon from Pokemon B1/W1 and his Pokemon will be level 60-68.
Chandelure Moveset: Energy ball, flamethrower, shadow ball, curse. Held Item: Sitrus berry.
Walrein Moveset: Ice fang, hail in case your ability is snow cloak or ice body), surf, body slam. Held Item: leftovers
Serperior Moveset: Coil, Leaf Blade, aqua tail, leaf storm. (Aqua tail can be learned by the move tutor in Lentimas Town and it is also for defending yourself against fire type Pokemon). Held Item: Meadow Plate.
Braviary Moveset: Brave bird, superpower(learned in Lentimas Town), fly, thrash. Held Item: Lum berry.
Mienshao Moveset: Hi jump kick, fake out, close combat and wide guard(for double and triples battles to protect your other Pokemon). Held Item: Fighting Gem.
Excadrill Moveset: Drill run, Earthquake, Swords Dance, Sandstorm.(In case your ability is either sane veil or sand rush). Held Item: Life Orb.