I never use Soothe Bell to the pokemons that evolve with trainers love and things like that. 10 of each vitamin like Protein, Carbos and others will be enough(they're a bit expensive too but in Battle Frontier they cost only 1BP). Just give 'em all to the pokemon you like to evolve and train it one level higher, and, Huh? [pokemon] is evolving. Congratulations, [pokemon] evolved into [pokemon].