you get this in the battle arena at the Gold Saucer. you have to grind out battle arena by winning all eight battles of a single session. the more rounds of a session you go the more you'll get debuffed in some way. the best debuffs to be granted in order to maximize battle points is the all materia broken debuff which by itself leaves you able to do basic attacks, limit breaks and items. you can trigger this trophy as early as right after getting the Tiny Bronco in the Disc 1 content but if you don't want to grind for 512K or more to get the Omnislash wait until the Disc 2 content where it gets discounted by quite a bit. I recommend using Cloud with both of his level 3 limit breaks unlocked since both can one-shot enemies in battle arena and the moment you earn the Omnislash item you can immediately use it to have Cloud learn it. but if you get mini or toad as debuffs you're better off quitting the current session since either of those make your damage output only 1 damage point per attack.