No need to wait till Volcarona, you can get Sun Stone in Nimbasa City and easily evolve Petilil at lv 26 to get Quiver Dance as Lilligant two levels later. I'm not sure about White 2 but in Black 2 Petilil is THE BEST Pokemon. Better than Rash Genesect, Timid Magnezone or Modest Starmie. Honestly.
Setting up with Sleep Powder is kinda risky as opposing mons often wake up next turn and you miss 25% of the time. Lilligant is fast enough to set up QD against 95% of the game (Timid, IVs, EVs). If you're against special attacker quite often not even super effective attacks will save him thanks to boost in SpDef. +1 Miracle Seed Petal Dance easily OHKOs resistors or special walls. She's unstoppable.