ENTER WITH LOTS OF BP!! there's nothing worse than being almost there but not finishing the job because of something as paltry as BP. Buy a lot from the shady guy at the cable car landing. Try doing it when Omnislash's value is halved (disc2-3), give Cloud Sprint Shoes, Slash-All for easy killings and near his limit. But don't use the limit till the final boss (the 8th) You can give him Mug as well so he can use that to attack if his bar fills. Equip some strong, effective magic just in case your weapon becomes sucky, something like Contain Magic or Ultima. Enemy Skill is vital (because of Big Guard, saves a lot of time). First battle don't kill your enemies yet, they're weak so use the time to cast Big Guard, Regen and Resist (not sure - auto-Esuna?). Keep your these 3 up, especially Big Guard and REGEN because if you're forced into a bad status Resist won't do anything against it. Keep yourself in good status with items, if you get in a petrify countdown use a soft. Actually it's all about just how much BP you hava and you'll get it eventually. The points just build up.

W-SUMMON tho is another story..yargh. But I figure if you can get Omnislash's points once you can do it twice, then you can get W-summon. Haha.