First, Victini is banned in all official formats. You could use it in free battles or local battles or whatever, but it would be pretty badly outclassed by stuff like Groudon and Ho-Oh. You could theoretically play local battles with OU rules, but I don't see the point in that when you can play the real OU on Showdown!. If you really want to play OU on a 3DS, then I guess I might as well help you. The standard OU set uses v-create, bolt strike, u-turn, and final gambit. The last two moves can be learned by your Victini, and the first two can be replaced with flare blitz and either thunder punch (for water coverage) or zen headbutt (for STAB). It should work, albeit be a little weaker.