I highly recommend you backtrack all the way to Besaid for the Jecht Sphere there after defeating Spherimorph, and you may as well get all the ones on the way too. If you don't, Dark Valefor will prevent you from returning to Besaid Village and you won't be able to defeat it without Yojimbo Zanmato abuse or heavy grinding, preventing you from getting the achievement and Auron's final Overdrive.