You don't specify if you're playing BW or BW2, but it doesn't matter because the answer is the same.
The best place to grind Litwick is Route 7 --- or more specifically, the Audino you find there. Audino at this level know exclusively Normal type moves, making it completely safe to Will-O-Wisp them (which Litwick already knows when you catch it). Flame Burst + steady burn damage will inevitably KO any Audino you encounter, and the Mistralton Pokemon Center is so close that refilling PP isn't a concern.
The Celestial Tower isn't as good as Route 7 because the Pokemon there give less experience and do more damage, making it an inefficient use of time.